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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Claims Development) (Details) | 12 Months Ended |  |  |  |  |  |  |
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Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2008 USD ($)
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Reserve for loss and loss expense | $ 3,771,240,000 | $ 3,691,719,000 |  |  |  |  |  |  |  |
Incurred loss and loss expense for claims occurring in the prior years | (39,192,000) | (65,768,000) | $ (69,009,000) |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 7,727,861,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 10,417,364,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 3,041,694,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2008 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 860,745,000 | 853,769,000 | 839,392,000 | $ 819,280,000 | $ 798,996,000 | $ 764,196,000 | $ 690,016,000 | $ 609,851,000 | $ 489,633,000 | $ 286,314,000
Net outstanding liabilities | 923,978,000 | 926,017,000 | 931,785,000 | 927,958,000 | 936,975,000 | 947,306,000 | 964,862,000 | 990,931,000 | 988,584,000 | 957,247,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 41,791,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 85,338 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2009 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 784,713,000 | 775,885,000 | 760,589,000 | 736,100,000 | 695,249,000 | 634,902,000 | 540,982,000 | 442,417,000 | 277,275,000 |
Net outstanding liabilities | 848,413,000 | 853,401,000 | 857,960,000 | 869,927,000 | 870,057,000 | 883,590,000 | 916,691,000 | 941,972,000 | 920,143,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 41,937,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 85,575 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2010 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 835,532,000 | 823,770,000 | 803,773,000 | 773,536,000 | 704,895,000 | 625,229,000 | 509,910,000 | 328,826,000 |  |
Net outstanding liabilities | 907,074,000 | 915,131,000 | 922,404,000 | 943,118,000 | 956,600,000 | 977,959,000 | 973,742,000 | 950,114,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 50,293,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 94,258 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2011 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 924,111,000 | 901,801,000 | 852,202,000 | 782,655,000 | 692,730,000 | 585,867,000 | 391,944,000 |  |  |
Net outstanding liabilities | 1,019,351,000 | 1,023,726,000 | 1,033,518,000 | 1,056,107,000 | 1,062,233,000 | 1,061,667,000 | 1,042,576,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 63,891,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 104,500 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2012 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 856,195,000 | 810,135,000 | 743,742,000 | 651,544,000 | 555,819,000 | 378,067,000 |  |  |  |
Net outstanding liabilities | 973,644,000 | 973,089,000 | 998,028,000 | 1,020,655,000 | 1,071,290,000 | 1,065,437,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 77,542,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 103,745 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2013 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 833,823,000 | 748,758,000 | 644,475,000 | 518,872,000 | 335,956,000 |  |  |  |  |
Net outstanding liabilities | 1,002,316,000 | 1,021,007,000 | 1,047,230,000 | 1,062,045,000 | 1,044,142,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 116,449,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 90,755 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2014 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 855,959,000 | 736,154,000 | 614,075,000 | 405,898,000 |  |  |  |  |  |
Net outstanding liabilities | 1,124,014,000 | 1,146,990,000 | 1,133,798,000 | 1,107,513,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 171,913,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 94,375 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2015 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 725,385,000 | 581,203,000 | 376,641,000 |  |  |  |  |  |  |
Net outstanding liabilities | 1,144,830,000 | 1,130,513,000 | 1,114,081,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 256,758,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 92,891 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2016 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 617,958,000 | 387,272,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,203,634,000 | 1,188,608,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 416,010,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 92,191 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2017 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 433,440,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,270,110,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 634,863,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 88,941 |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | $ 352,192,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,262,217,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 2,273,761,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 1,092,058,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,272,395,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,941,745,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 916,471,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,677,641,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 2,137,614,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 464,131,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 401,363,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 477,234,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 83,163,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 1,080,689,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,130,739,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 50,300,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 198,360,000 | 194,257,000 | 186,896,000 | 176,873,000 | 166,394,000 | 151,055,000 | 113,088,000 | 82,421,000 | 45,595,000 | 16,397,000
Net outstanding liabilities | 219,617,000 | 221,203,000 | 221,640,000 | 219,551,000 | 224,236,000 | 233,712,000 | 234,770,000 | 243,536,000 | 243,755,000 | 250,239,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 14,326,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 13,769,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 198,512,000 | 195,962,000 | 191,265,000 | 188,206,000 | 180,556,000 | 171,403,000 | 158,083,000 | 139,144,000 | 100,678,000 | 39,628,000
Net outstanding liabilities | 238,218,000 | 238,836,000 | 243,512,000 | 244,100,000 | 245,993,000 | 241,921,000 | 250,423,000 | 255,810,000 | 243,186,000 | 219,616,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 25,993,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 14,401,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 181,979,000 | 181,779,000 | 180,779,000 | 175,244,000 | 173,795,000 | 166,487,000 | 151,741,000 | 130,857,000 | 104,711,000 | 69,053,000
Net outstanding liabilities | 183,151,000 | 183,618,000 | 183,736,000 | 180,902,000 | 182,785,000 | 187,417,000 | 187,100,000 | 190,349,000 | 195,823,000 | 196,370,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 736,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 24,129 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 30,873,000 | 30,873,000 | 29,778,000 | 28,589,000 | 28,660,000 | 28,026,000 | 25,738,000 | 24,449,000 | 21,765,000 | 15,019,000
Net outstanding liabilities | 31,065,000 | 30,967,000 | 31,340,000 | 31,066,000 | 29,793,000 | 30,250,000 | 32,974,000 | 39,334,000 | 38,986,000 | 39,660,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 192,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,258,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 101,032,000 | 101,034,000 | 100,868,000 | 100,908,000 | 100,752,000 | 100,213,000 | 99,288,000 | 100,465,000 | 98,921,000 | 68,211,000
Net outstanding liabilities | 101,054,000 | 101,043,000 | 100,881,000 | 101,043,000 | 101,702,000 | 101,265,000 | 101,470,000 | 101,436,000 | 102,860,000 | 97,578,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 2,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 7,604,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 180,621,000 | 176,316,000 | 166,767,000 | 151,920,000 | 130,554,000 | 103,213,000 | 64,970,000 | 37,143,000 | 14,346,000 |
Net outstanding liabilities | 201,568,000 | 205,741,000 | 206,387,000 | 211,243,000 | 212,947,000 | 223,146,000 | 233,530,000 | 241,625,000 | 237,913,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 17,629,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 13,841,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 167,894,000 | 164,336,000 | 160,529,000 | 154,726,000 | 145,417,000 | 133,116,000 | 117,019,000 | 87,299,000 | 37,885,000 |
Net outstanding liabilities | 208,142,000 | 208,611,000 | 212,536,000 | 216,992,000 | 210,756,000 | 210,109,000 | 213,036,000 | 215,946,000 | 197,504,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 22,575,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 12,217,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 160,013,000 | 159,723,000 | 158,303,000 | 155,560,000 | 149,949,000 | 137,564,000 | 113,697,000 | 94,406,000 | 63,126,000 |
Net outstanding liabilities | 161,300,000 | 161,923,000 | 161,251,000 | 162,911,000 | 166,682,000 | 169,858,000 | 182,724,000 | 191,079,000 | 199,541,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 897,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 24,652 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 43,547,000 | 43,448,000 | 43,358,000 | 42,895,000 | 40,052,000 | 36,073,000 | 32,689,000 | 29,612,000 | 18,915,000 |
Net outstanding liabilities | 43,933,000 | 44,273,000 | 44,299,000 | 44,938,000 | 43,553,000 | 43,828,000 | 46,645,000 | 51,762,000 | 48,535,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 272,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,474,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 80,405,000 | 80,509,000 | 80,529,000 | 80,352,000 | 80,251,000 | 80,894,000 | 80,433,000 | 78,695,000 | 59,933,000 |
Net outstanding liabilities | 80,416,000 | 80,545,000 | 80,558,000 | 80,455,000 | 80,774,000 | 82,014,000 | 82,025,000 | 82,124,000 | 82,619,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 4,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 7,009,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 178,179,000 | 172,394,000 | 161,487,000 | 143,360,000 | 113,050,000 | 80,018,000 | 46,201,000 | 15,726,000 |  |
Net outstanding liabilities | 202,394,000 | 208,968,000 | 211,619,000 | 222,328,000 | 237,154,000 | 242,499,000 | 228,680,000 | 215,208,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 18,403,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 12,672,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 162,796,000 | 158,078,000 | 153,795,000 | 149,086,000 | 137,184,000 | 122,442,000 | 93,281,000 | 46,795,000 |  |
Net outstanding liabilities | 204,423,000 | 208,155,000 | 212,448,000 | 214,916,000 | 211,030,000 | 212,815,000 | 214,469,000 | 198,371,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 28,964,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 12,184,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 169,793,000 | 169,100,000 | 167,227,000 | 163,513,000 | 146,913,000 | 128,015,000 | 99,254,000 | 68,098,000 |  |
Net outstanding liabilities | 173,471,000 | 173,157,000 | 172,969,000 | 179,854,000 | 181,923,000 | 187,778,000 | 189,305,000 | 187,562,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 1,491,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 25,301 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 40,395,000 | 40,279,000 | 38,900,000 | 37,819,000 | 34,705,000 | 31,027,000 | 28,131,000 | 20,821,000 |  |
Net outstanding liabilities | 41,197,000 | 41,239,000 | 40,581,000 | 40,899,000 | 39,915,000 | 42,408,000 | 49,285,000 | 53,669,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 697,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,917,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 95,150,000 | 95,156,000 | 95,147,000 | 95,270,000 | 95,111,000 | 94,602,000 | 91,918,000 | 69,543,000 |  |
Net outstanding liabilities | 95,155,000 | 95,178,000 | 95,363,000 | 95,530,000 | 96,127,000 | 97,386,000 | 96,851,000 | 105,647,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 5,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 7,667,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 170,525,000 | 159,768,000 | 135,377,000 | 102,978,000 | 73,643,000 | 42,692,000 | 13,924,000 |  |  |
Net outstanding liabilities | 212,011,000 | 211,196,000 | 217,256,000 | 230,785,000 | 239,480,000 | 228,720,000 | 227,769,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 25,729,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 11,579,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 154,320,000 | 149,269,000 | 139,232,000 | 134,646,000 | 118,847,000 | 90,836,000 | 42,941,000 |  |  |
Net outstanding liabilities | 200,674,000 | 205,708,000 | 210,591,000 | 215,114,000 | 214,743,000 | 218,973,000 | 205,238,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 32,881,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 11,845,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 170,000,000 | 166,082,000 | 157,291,000 | 142,507,000 | 121,576,000 | 99,196,000 | 69,849,000 |  |  |
Net outstanding liabilities | 174,514,000 | 174,882,000 | 172,617,000 | 178,421,000 | 182,325,000 | 183,044,000 | 174,006,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 2,937,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 25,272 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 57,045,000 | 55,856,000 | 52,114,000 | 46,444,000 | 41,011,000 | 37,362,000 | 27,884,000 |  |  |
Net outstanding liabilities | 58,456,000 | 58,966,000 | 59,256,000 | 58,242,000 | 51,047,000 | 57,083,000 | 54,469,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 1,080,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 4,959,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 131,089,000 | 131,115,000 | 131,060,000 | 130,681,000 | 129,579,000 | 127,580,000 | 94,538,000 |  |  |
Net outstanding liabilities | 131,049,000 | 131,113,000 | 131,353,000 | 131,282,000 | 130,942,000 | 131,667,000 | 136,954,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 4,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 9,036,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 130,866,000 | 109,448,000 | 89,008,000 | 56,580,000 | 35,241,000 | 13,030,000 |  |  |  |
Net outstanding liabilities | 175,268,000 | 175,305,000 | 194,144,000 | 215,083,000 | 245,561,000 | 238,979,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 27,702,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 9,922,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 139,477,000 | 132,052,000 | 122,755,000 | 108,211,000 | 86,909,000 | 40,911,000 |  |  |  |
Net outstanding liabilities | 187,359,000 | 188,596,000 | 195,197,000 | 199,360,000 | 208,036,000 | 203,864,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 36,233,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 11,605,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 176,656,000 | 168,114,000 | 148,669,000 | 127,235,000 | 105,371,000 | 73,316,000 |  |  |  |
Net outstanding liabilities | 186,128,000 | 184,367,000 | 184,289,000 | 183,527,000 | 191,947,000 | 179,551,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 3,880,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 23,889 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 40,627,000 | 38,766,000 | 37,215,000 | 35,089,000 | 31,833,000 | 22,199,000 |  |  |  |
Net outstanding liabilities | 43,747,000 | 44,077,000 | 44,172,000 | 46,303,000 | 48,029,000 | 54,342,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 756,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 5,540,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 116,625,000 | 116,291,000 | 114,699,000 | 111,503,000 | 108,834,000 | 81,528,000 |  |  |  |
Net outstanding liabilities | 117,170,000 | 117,102,000 | 116,658,000 | 115,375,000 | 114,224,000 | 118,464,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 24,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 8,514,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 139,114,000 | 104,587,000 | 72,127,000 | 35,113,000 | 12,789,000 |  |  |  |  |
Net outstanding liabilities | 210,785,000 | 225,709,000 | 239,776,000 | 251,421,000 | 250,609,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 53,014,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 10,226,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 118,669,000 | 109,739,000 | 96,376,000 | 74,568,000 | 36,829,000 |  |  |  |  |
Net outstanding liabilities | 166,662,000 | 173,160,000 | 187,658,000 | 194,318,000 | 199,794,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 34,776,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 11,366,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 189,626,000 | 169,850,000 | 140,015,000 | 109,893,000 | 76,469,000 |  |  |  |  |
Net outstanding liabilities | 210,410,000 | 207,994,000 | 209,197,000 | 205,282,000 | 188,289,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 8,258,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 25,392 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 37,993,000 | 34,760,000 | 30,845,000 | 26,592,000 | 17,412,000 |  |  |  |  |
Net outstanding liabilities | 41,369,000 | 40,624,000 | 41,005,000 | 42,618,000 | 49,617,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 3,192,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,479,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 90,840,000 | 90,350,000 | 90,446,000 | 87,874,000 | 60,244,000 |  |  |  |  |
Net outstanding liabilities | 90,436,000 | 90,005,000 | 90,103,000 | 90,639,000 | 88,101,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 42,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 5,709,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 121,969,000 | 79,972,000 | 46,825,000 | 14,901,000 |  |  |  |  |  |
Net outstanding liabilities | 239,333,000 | 257,132,000 | 249,946,000 | 244,312,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 80,168,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 10,391,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 113,626,000 | 100,876,000 | 78,944,000 | 35,924,000 |  |  |  |  |  |
Net outstanding liabilities | 172,515,000 | 182,579,000 | 187,065,000 | 199,346,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 42,869,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 10,482,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 180,701,000 | 148,884,000 | 117,169,000 | 80,810,000 |  |  |  |  |  |
Net outstanding liabilities | 219,925,000 | 216,824,000 | 212,725,000 | 200,534,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 19,053,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 27,338 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 49,460,000 | 44,911,000 | 40,584,000 | 28,914,000 |  |  |  |  |  |
Net outstanding liabilities | 59,806,000 | 62,548,000 | 60,949,000 | 55,962,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 5,952,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 4,054,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 137,883,000 | 136,634,000 | 132,909,000 | 101,131,000 |  |  |  |  |  |
Net outstanding liabilities | 138,751,000 | 136,820,000 | 136,249,000 | 141,192,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 126,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 6,512,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 78,668,000 | 39,978,000 | 14,665,000 |  |  |  |  |  |  |
Net outstanding liabilities | 246,990,000 | 245,710,000 | 254,720,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 124,639,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 9,987,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 98,195,000 | 77,320,000 | 33,857,000 |  |  |  |  |  |  |
Net outstanding liabilities | 183,604,000 | 194,639,000 | 193,729,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 42,287,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 10,530,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 175,866,000 | 132,260,000 | 91,347,000 |  |  |  |  |  |  |
Net outstanding liabilities | 253,074,000 | 240,958,000 | 220,994,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 36,137,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 28,818 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 42,710,000 | 36,014,000 | 24,189,000 |  |  |  |  |  |  |
Net outstanding liabilities | 57,245,000 | 53,768,000 | 52,871,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 10,256,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,913,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 109,829,000 | 106,182,000 | 79,048,000 |  |  |  |  |  |  |
Net outstanding liabilities | 111,750,000 | 109,513,000 | 110,270,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 261,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 6,398,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 46,549,000 | 15,684,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 272,048,000 | 277,214,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 178,904,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 9,617,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 78,531,000 | 34,525,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 184,946,000 | 196,774,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 70,424,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 10,509,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 155,720,000 | 106,022,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 274,367,000 | 255,187,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 71,303,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 30,480 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 36,848,000 | 24,655,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 53,792,000 | 52,335,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 11,938,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,757,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 118,789,000 | 83,966,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 126,185,000 | 121,927,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 804,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 6,686,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 17,366,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 293,747,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 249,085,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 8,194,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 40,375,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 195,202,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 112,086,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 10,182,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 117,287,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 301,274,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 132,814,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 30,187 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 21,865,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 46,624,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 15,252,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 3,462,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 99,047,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 138,773,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 8,794,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 6,358,000,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | $ 80,514,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 247,121,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 4,158,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 7,292,000 |  |  |  |  |  |  |  |  |
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 250,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 962,602,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,049,297,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 92,557,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 648,479,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 673,804,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 30,728,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 98,810,000 | 98,668,000 | 98,604,000 | 98,174,000 | 97,723,000 | 95,932,000 | 91,834,000 | 84,715,000 | 73,194,000 | 50,396,000
Net outstanding liabilities | 99,270,000 | 99,116,000 | 99,255,000 | 99,686,000 | 99,912,000 | 100,213,000 | 103,934,000 | 106,842,000 | 106,999,000 | 100,311,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 191,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 16,042 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 37,928,000 | 37,930,000 | 37,939,000 | 37,933,000 | 37,837,000 | 37,763,000 | 37,086,000 | 36,271,000 | 33,535,000 | 21,277,000
Net outstanding liabilities | 38,038,000 | 38,045,000 | 38,042,000 | 38,111,000 | 38,410,000 | 38,062,000 | 39,203,000 | 39,342,000 | 41,747,000 | $ 41,224,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 56,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 5,139,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 103,009,000 | 102,437,000 | 102,322,000 | 102,187,000 | 100,566,000 | 96,229,000 | 86,431,000 | 71,911,000 | 51,039,000 |
Net outstanding liabilities | 103,348,000 | 103,412,000 | 103,393,000 | 103,866,000 | 104,734,000 | 103,908,000 | 105,033,000 | 103,319,000 | 93,808,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 191,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 17,346 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 40,269,000 | 40,189,000 | 39,907,000 | 39,819,000 | 39,731,000 | 39,342,000 | 38,078,000 | 36,965,000 | 28,299,000 |
Net outstanding liabilities | 40,451,000 | 40,457,000 | 40,465,000 | 40,400,000 | 61,927,000 | 40,313,000 | 42,609,000 | 44,511,000 | $ 47,636,000 |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 73,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 5,633,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 106,733,000 | 106,453,000 | 105,849,000 | 104,061,000 | 101,540,000 | 95,300,000 | 82,490,000 | 58,786,000 |  |
Net outstanding liabilities | 107,054,000 | 107,224,000 | 107,405,000 | 107,490,000 | 109,515,000 | 112,346,000 | 110,075,000 | 103,340,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 222,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 20,822 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 62,272,000 | 62,241,000 | 62,227,000 | 62,155,000 | 61,106,000 | 60,295,000 | 58,638,000 | 43,699,000 |  |
Net outstanding liabilities | 62,392,000 | 62,339,000 | 62,402,000 | 62,462,000 | 97,761,000 | 63,285,000 | 67,525,000 | $ 68,373,000 |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 84,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 9,131,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 113,551,000 | 112,732,000 | 111,085,000 | 105,068,000 | 93,878,000 | 82,102,000 | 61,323,000 |  |  |
Net outstanding liabilities | 113,988,000 | 113,830,000 | 114,241,000 | 112,993,000 | 113,686,000 | 116,164,000 | 113,232,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 284,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 22,700 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 94,007,000 | 93,720,000 | 93,312,000 | 92,185,000 | 91,718,000 | 89,963,000 | 71,668,000 |  |  |
Net outstanding liabilities | 94,378,000 | 94,183,000 | 94,543,000 | 94,167,000 | 82,744,000 | 98,211,000 | 103,804,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 159,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 15,106,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 109,355,000 | 107,890,000 | 102,977,000 | 94,842,000 | 82,729,000 | 63,704,000 |  |  |  |
Net outstanding liabilities | 110,300,000 | 110,294,000 | 109,324,000 | 109,832,000 | 114,921,000 | 113,771,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 728,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 22,332 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 85,562,000 | 85,196,000 | 84,250,000 | 82,720,000 | 79,584,000 | 69,056,000 |  |  |  |
Net outstanding liabilities | 86,330,000 | 86,271,000 | 86,667,000 | 86,560,000 | 82,745,000 | 87,260,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 180,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 16,936,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 105,131,000 | 100,528,000 | 92,637,000 | 80,861,000 | 61,384,000 |  |  |  |  |
Net outstanding liabilities | 107,759,000 | 106,703,000 | 106,225,000 | 109,620,000 | 108,417,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 851,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 22,371 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 71,776,000 | 69,775,000 | 67,838,000 | 65,528,000 | 50,664,000 |  |  |  |  |
Net outstanding liabilities | 71,714,000 | 72,145,000 | 71,494,000 | 72,528,000 | 73,670,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 284,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 7,747,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 99,173,000 | 92,589,000 | 83,739,000 | 62,519,000 |  |  |  |  |  |
Net outstanding liabilities | 107,452,000 | 106,757,000 | 109,325,000 | 102,250,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 2,554,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 22,499 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 81,664,000 | 79,751,000 | 76,007,000 | 61,561,000 |  |  |  |  |  |
Net outstanding liabilities | 83,844,000 | 83,637,000 | 82,461,000 | 80,111,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 1,146,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 8,762,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 87,163,000 | 76,470,000 | 58,725,000 |  |  |  |  |  |  |
Net outstanding liabilities | 100,214,000 | 99,698,000 | 96,387,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 6,541,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 20,840 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 72,202,000 | 70,078,000 | 52,589,000 |  |  |  |  |  |  |
Net outstanding liabilities | 76,559,000 | 76,400,000 | 76,637,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 2,744,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 7,724,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 76,823,000 | 57,961,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 98,032,000 | 92,727,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 11,651,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 19,747 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 57,333,000 | 42,252,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 60,931,000 | 60,105,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 2,067,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 6,820,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 62,854,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 101,880,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 22,284,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 19,818 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 45,466,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 59,167,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 5,315,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 6,651,000,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | $ 5,862,000 |  |  |  |  |  |  |  |  |
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 5,403,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Reserve for loss and loss expense |  |  |  |  |  | 15,000,000 |  |  |  |
E&S Lines [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 192,604,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 471,628,000 |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 279,023,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Commercial Property [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 13,519,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | 97,000 | 92,000 | 79,000 | 80,000 | 70,000 | 24,000 | 0 |  |  |
Net outstanding liabilities | 119,000 | (162,000) | 52,000 | 119,000 | 146,000 | 169,000 | 92,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 0 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 35,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 737,000 | 709,000 | 626,000 | 605,000 | 431,000 | 198,000 | 0 |  |  |
Net outstanding liabilities | 737,000 | 96,000 | 710,000 | 728,000 | 938,000 | 1,053,000 | 885,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 0 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 274,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 4,610,000 | 4,513,000 | 4,078,000 | 3,574,000 | 2,570,000 | 1,218,000 | 0 |  |  |
Net outstanding liabilities | 4,932,000 | 3,055,000 | 3,831,000 | 4,299,000 | 3,369,000 | 4,106,000 | 3,294,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 0 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 804,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 10,256,000 | 9,912,000 | 9,954,000 | 6,445,000 | 3,200,000 | 806,000 | 0 |  |  |
Net outstanding liabilities | 10,228,000 | 9,652,000 | 10,273,000 | 12,207,000 | 9,853,000 | 7,102,000 | $ 8,127,000 |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 361,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 1,316,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 36,278,000 | 32,343,000 | 25,064,000 | 16,430,000 | 7,914,000 | 3,722,000 |  |  |  |
Net outstanding liabilities | 45,988,000 | 46,165,000 | 46,149,000 | 43,175,000 | 42,621,000 | $ 42,367,000 |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 7,313,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 2,006,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 46,108,000 | 35,200,000 | 21,980,000 | 9,470,000 | 2,715,000 |  |  |  |  |
Net outstanding liabilities | 67,647,000 | 69,112,000 | 67,099,000 | 60,309,000 | $ 55,468,000 |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 15,326,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 2,219,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 43,877,000 | 25,571,000 | 12,234,000 | 2,353,000 |  |  |  |  |  |
Net outstanding liabilities | 71,719,000 | 69,929,000 | 63,505,000 | $ 55,316,000 |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 18,224,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 1,987,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 29,389,000 | 13,057,000 | 3,036,000 |  |  |  |  |  |  |
Net outstanding liabilities | 82,404,000 | 76,432,000 | $ 75,498,000 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 32,909,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 2,606,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 16,195,000 | 3,720,000 |  |  |  |  |  |  |  |
Net outstanding liabilities | 96,416,000 | $ 94,451,000 |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 65,489,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 2,454,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net | $ 5,057,000 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 91,438,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net | $ 79,354,000 |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Number of Reported Claims | 1,760,000,000 |  |  |  |  |  |  |  |  |
E&S Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2008 [Member] | General Liability [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | $ 0 |  |  |  |  |  |  |  |  |

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