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Reserve for Losses and Loss Expenses Reserve for Losses and Loss Expenses (Claims Development) (Details)
 12 Months Ended
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 ($)
Dec. 31, 2007
USD ($)
Claims Development [Line Items]                    
Reserve for losses and loss expenses $ 3,691,719,000 $ 3,517,728,000                
Incurred losses and loss expenses for claims occurring in the prior years (65,768,000) (69,009,000) $ (59,269,000)              
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 7,547,829,000                  
Net outstanding liabilities 10,168,191,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 2,944,432,000                  
Short-duration Insurance Contracts, Accident Year 2007 [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 929,082,000 920,931,000 912,683,000 $ 896,590,000 $ 877,933,000 $ 835,347,000 $ 762,422,000 $ 665,277,000 $ 543,949,000 $ 350,369,000
Net outstanding liabilities 989,709,000 992,673,000 998,496,000 1,003,552,000 1,015,897,000 1,028,956,000 1,028,546,000 1,047,912,000 1,066,670,000 1,038,585,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 42,970,000                  
Short-duration Insurance Contracts, Number of Reported Claims 84,996                  
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 $ 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 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 $ 48,590,000                  
Short-duration Insurance Contracts, Number of Reported Claims 85,264                  
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 $ 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 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 $ 49,532,000                  
Short-duration Insurance Contracts, Number of Reported Claims 85,444                  
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 $ 823,770,000 803,773,000 773,536,000 704,895,000 625,229,000 509,910,000 328,826,000      
Net outstanding liabilities 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 $ 65,625,000                  
Short-duration Insurance Contracts, Number of Reported Claims 94,093                  
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 $ 901,801,000 852,202,000 782,655,000 692,730,000 585,867,000 391,944,000        
Net outstanding liabilities 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 $ 82,565,000                  
Short-duration Insurance Contracts, Number of Reported Claims 104,303                  
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 $ 810,135,000 743,742,000 651,544,000 555,819,000 378,067,000          
Net outstanding liabilities 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 $ 101,992,000                  
Short-duration Insurance Contracts, Number of Reported Claims 103,498                  
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 $ 748,758,000 644,475,000 518,872,000 335,956,000            
Net outstanding liabilities 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 $ 182,613,000                  
Short-duration Insurance Contracts, Number of Reported Claims 90,330                  
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 $ 736,154,000 614,075,000 405,898,000              
Net outstanding liabilities 1,146,990,000 1,133,798,000 1,107,513,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 278,689,000                  
Short-duration Insurance Contracts, Number of Reported Claims 93,747                  
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 $ 581,203,000 376,641,000                
Net outstanding liabilities 1,130,513,000 1,114,081,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 375,894,000                  
Short-duration Insurance Contracts, Number of Reported Claims 91,410                  
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 $ 387,272,000                  
Net outstanding liabilities 1,188,608,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 589,938,000                  
Short-duration Insurance Contracts, Number of Reported Claims 85,202                  
Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net $ 324,070,000                  
Standard Commercial Lines [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,260,853,000                  
Net outstanding liabilities 2,258,895,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 1,070,929,000                  
Standard Commercial Lines [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,303,022,000                  
Net outstanding liabilities 2,013,235,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 936,766,000                  
Standard Commercial Lines [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,575,794,000                  
Net outstanding liabilities 1,974,804,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 402,281,000                  
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 378,610,000                  
Net outstanding liabilities 458,085,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 86,802,000                  
Standard Commercial Lines [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,044,298,000                  
Net outstanding liabilities 1,086,323,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 42,279,000                  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 208,449,000 204,159,000 201,842,000 191,405,000 181,641,000 158,424,000 123,108,000 80,621,000 44,356,000 14,695,000
Net outstanding liabilities 230,717,000 231,439,000 234,697,000 234,173,000 242,878,000 248,807,000 250,834,000 255,538,000 256,627,000 252,732,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 17,815,000                  
Short-duration Insurance Contracts, Number of Reported Claims 14,016                  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 180,865,000 179,011,000 175,205,000 169,941,000 164,373,000 156,320,000 144,713,000 123,681,000 88,786,000 31,478,000
Net outstanding liabilities 216,177,000 218,828,000 222,623,000 225,904,000 225,165,000 230,109,000 226,095,000 231,104,000 236,993,000 231,462,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 23,152,000                  
Short-duration Insurance Contracts, Number of Reported Claims 16,344                  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 173,980,000 173,050,000 171,839,000 171,622,000 170,241,000 163,629,000 146,690,000 126,507,000 102,233,000 65,544,000
Net outstanding liabilities 175,894,000 175,431,000 175,342,000 176,049,000 178,956,000 179,739,000 182,030,000 187,966,000 194,567,000 185,733,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,434,000                  
Short-duration Insurance Contracts, Number of Reported Claims 24,074                  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 29,048,000 28,368,000 28,095,000 28,073,000 27,273,000 25,759,000 24,978,000 22,583,000 18,870,000 14,051,000
Net outstanding liabilities 29,288,000 29,691,000 28,503,000 28,440,000 29,393,000 29,270,000 31,212,000 33,397,000 34,011,000 32,749,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 124,000                  
Short-duration Insurance Contracts, Number of Reported Claims 2,956                  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 103,061,000 102,663,000 102,532,000 102,370,000 102,310,000 101,805,000 99,942,000 99,036,000 95,050,000 63,159,000
Net outstanding liabilities 103,077,000 102,679,000 102,732,000 102,998,000 103,381,000 103,183,000 101,027,000 100,809,000 104,160,000 98,167,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,000                  
Short-duration Insurance Contracts, Number of Reported Claims 6,919                  
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 $ 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 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 $ 19,939,000                  
Short-duration Insurance Contracts, Number of Reported Claims 13,721                  
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 $ 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 238,836,000 243,512,000 244,100,000 245,993,000 241,921,000 250,423,000 255,810,000 243,186,000 219,616,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 26,983,000                  
Short-duration Insurance Contracts, Number of Reported Claims 14,400                  
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 $ 181,779,000 180,779,000 175,244,000 173,795,000 166,487,000 151,741,000 130,857,000 104,711,000 69,053,000  
Net outstanding liabilities 183,618,000 183,736,000 180,902,000 182,785,000 187,417,000 187,100,000 190,349,000 195,823,000 196,370,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,332,000                  
Short-duration Insurance Contracts, Number of Reported Claims 24,105                  
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 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 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 $ 94,000                  
Short-duration Insurance Contracts, Number of Reported Claims 3,258                  
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 $ 101,034,000 100,868,000 100,908,000 100,752,000 100,213,000 99,288,000 100,465,000 98,921,000 68,211,000  
Net outstanding liabilities 101,043,000 100,881,000 101,043,000 101,702,000 101,265,000 101,470,000 101,436,000 102,860,000 97,578,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,000                  
Short-duration Insurance Contracts, Number of Reported Claims 7,604                  
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 $ 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 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 $ 22,858,000                  
Short-duration Insurance Contracts, Number of Reported Claims 13,815                  
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 $ 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 208,611,000 212,536,000 216,992,000 210,756,000 210,109,000 213,036,000 215,946,000 197,504,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 24,238,000                  
Short-duration Insurance Contracts, Number of Reported Claims 12,214                  
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 $ 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 161,923,000 161,251,000 162,911,000 166,682,000 169,858,000 182,724,000 191,079,000 199,541,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,873,000                  
Short-duration Insurance Contracts, Number of Reported Claims 24,554                  
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 $ 43,448,000 43,358,000 42,895,000 40,052,000 36,073,000 32,689,000 29,612,000 18,915,000    
Net outstanding liabilities 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 $ 730,000                  
Short-duration Insurance Contracts, Number of Reported Claims 3,473                  
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 $ 80,509,000 80,529,000 80,352,000 80,251,000 80,894,000 80,433,000 78,695,000 59,933,000    
Net outstanding liabilities 80,545,000 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 $ 10,000                  
Short-duration Insurance Contracts, Number of Reported Claims 7,009                  
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 $ 172,394,000 161,487,000 143,360,000 113,050,000 80,018,000 46,201,000 15,726,000      
Net outstanding liabilities 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 $ 29,380,000                  
Short-duration Insurance Contracts, Number of Reported Claims 12,629                  
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 $ 158,078,000 153,795,000 149,086,000 137,184,000 122,442,000 93,281,000 46,795,000      
Net outstanding liabilities 208,155,000 212,448,000 214,916,000 211,030,000 212,838,000 214,469,000 198,371,000      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 34,437,000                  
Short-duration Insurance Contracts, Number of Reported Claims 12,181                  
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 $ 169,100,000 167,227,000 163,513,000 146,913,000 128,015,000 99,254,000 68,098,000      
Net outstanding liabilities 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 $ 2,318,000                  
Short-duration Insurance Contracts, Number of Reported Claims 25,194                  
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 $ 40,279,000 38,900,000 37,819,000 34,705,000 31,027,000 28,131,000 20,821,000      
Net outstanding liabilities 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 $ 693,000                  
Short-duration Insurance Contracts, Number of Reported Claims 3,917                  
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 $ 95,156,000 95,147,000 95,270,000 95,111,000 94,602,000 91,918,000 69,543,000      
Net outstanding liabilities 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 $ 21,000                  
Short-duration Insurance Contracts, Number of Reported Claims 7,667                  
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 $ 159,768,000 135,377,000 102,978,000 73,643,000 42,692,000 13,924,000        
Net outstanding liabilities 211,196,000 217,256,000 230,785,000 239,480,000 228,720,000 229,967,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 36,350,000                  
Short-duration Insurance Contracts, Number of Reported Claims 11,533                  
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 $ 149,269,000 139,232,000 134,646,000 118,847,000 90,836,000 42,941,000        
Net outstanding liabilities 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 $ 38,227,000                  
Short-duration Insurance Contracts, Number of Reported Claims 11,843                  
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 $ 166,082,000 157,291,000 142,507,000 121,576,000 99,196,000 69,849,000        
Net outstanding liabilities 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 $ 5,153,000                  
Short-duration Insurance Contracts, Number of Reported Claims 25,146                  
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 $ 55,856,000 52,114,000 46,444,000 41,011,000 37,362,000 27,884,000        
Net outstanding liabilities 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 $ 2,177,000                  
Short-duration Insurance Contracts, Number of Reported Claims 4,956                  
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 $ 131,115,000 131,060,000 130,681,000 129,579,000 127,580,000 94,538,000        
Net outstanding liabilities 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 $ 22,000                  
Short-duration Insurance Contracts, Number of Reported Claims 9,035                  
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 $ 109,448,000 89,008,000 56,580,000 35,241,000 13,030,000          
Net outstanding liabilities 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 $ 44,493,000                  
Short-duration Insurance Contracts, Number of Reported Claims 9,864                  
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 $ 132,052,000 122,755,000 108,211,000 86,909,000 40,911,000          
Net outstanding liabilities 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 $ 39,122,000                  
Short-duration Insurance Contracts, Number of Reported Claims 11,601                  
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 $ 168,114,000 148,669,000 127,235,000 105,371,000 73,316,000          
Net outstanding liabilities 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 $ 6,421,000                  
Short-duration Insurance Contracts, Number of Reported Claims 23,751                  
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 $ 38,766,000 37,215,000 35,089,000 31,833,000 22,199,000          
Net outstanding liabilities 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 $ 834,000                  
Short-duration Insurance Contracts, Number of Reported Claims 5,533                  
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 $ 116,291,000 114,699,000 111,503,000 108,834,000 81,528,000          
Net outstanding liabilities 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 $ (22,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 8,512                  
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 $ 104,587,000 72,127,000 35,113,000 12,789,000            
Net outstanding liabilities 225,709,000 239,776,000 251,421,000 250,609,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 90,026,000                  
Short-duration Insurance Contracts, Number of Reported Claims 10,107                  
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 $ 109,739,000 96,376,000 74,568,000 36,829,000            
Net outstanding liabilities 173,160,000 187,658,000 194,318,000 199,794,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 43,058,000                  
Short-duration Insurance Contracts, Number of Reported Claims 11,361                  
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 $ 169,850,000 140,015,000 109,893,000 76,469,000            
Net outstanding liabilities 207,994,000 209,197,000 205,282,000 188,289,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 18,464,000                  
Short-duration Insurance Contracts, Number of Reported Claims 25,215                  
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 $ 34,760,000 30,845,000 26,592,000 17,412,000            
Net outstanding liabilities 40,624,000 41,005,000 42,618,000 49,617,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,189,000                  
Short-duration Insurance Contracts, Number of Reported Claims 3,474                  
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 $ 90,350,000 90,446,000 87,874,000 60,244,000            
Net outstanding liabilities 90,005,000 90,103,000 90,639,000 88,101,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (78,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 5,704                  
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 $ 79,972,000 46,825,000 14,901,000              
Net outstanding liabilities 257,132,000 249,946,000 244,312,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 135,883,000                  
Short-duration Insurance Contracts, Number of Reported Claims 10,157                  
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 $ 100,876,000 78,944,000 35,924,000              
Net outstanding liabilities 182,579,000 187,065,000 199,346,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 55,599,000                  
Short-duration Insurance Contracts, Number of Reported Claims 10,464                  
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 $ 148,884,000 117,169,000 80,810,000              
Net outstanding liabilities 216,824,000 212,725,000 200,534,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 37,432,000                  
Short-duration Insurance Contracts, Number of Reported Claims 27,129                  
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 $ 44,911,000 40,584,000 28,914,000              
Net outstanding liabilities 62,548,000 60,949,000 55,962,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 10,891,000                  
Short-duration Insurance Contracts, Number of Reported Claims 4,038                  
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 $ 136,634,000 132,909,000 101,131,000              
Net outstanding liabilities 136,820,000 136,249,000 141,192,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (1,052,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 6,503                  
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 $ 39,978,000 14,665,000                
Net outstanding liabilities 245,710,000 254,720,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 167,995,000                  
Short-duration Insurance Contracts, Number of Reported Claims 9,371                  
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 $ 77,320,000 33,857,000                
Net outstanding liabilities 194,639,000 193,729,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 63,496,000                  
Short-duration Insurance Contracts, Number of Reported Claims 10,479                  
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 $ 132,260,000 91,347,000                
Net outstanding liabilities 240,958,000 220,994,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 65,528,000                  
Short-duration Insurance Contracts, Number of Reported Claims 28,475                  
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 $ 36,014,000 24,189,000                
Net outstanding liabilities 53,768,000 52,871,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 12,089,000                  
Short-duration Insurance Contracts, Number of Reported Claims 3,860                  
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 $ 106,182,000 79,048,000                
Net outstanding liabilities 109,513,000 110,270,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (1,320,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 6,380                  
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 $ 15,684,000                  
Net outstanding liabilities 277,214,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 233,794,000                  
Short-duration Insurance Contracts, Number of Reported Claims 7,790                  
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 $ 34,525,000                  
Net outstanding liabilities 196,774,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 107,977,000                  
Short-duration Insurance Contracts, Number of Reported Claims 9,910                  
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 $ 106,022,000                  
Net outstanding liabilities 255,187,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 106,894,000                  
Short-duration Insurance Contracts, Number of Reported Claims 28,740                  
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 $ 24,655,000                  
Net outstanding liabilities 52,335,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 16,027,000                  
Short-duration Insurance Contracts, Number of Reported Claims 3,398                  
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 $ 83,966,000                  
Net outstanding liabilities 121,927,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 7,112,000                  
Short-duration Insurance Contracts, Number of Reported Claims 6,253                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net $ 72,887,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 226,553,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 3,271,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 7,327,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 254,000                  
Standard Personal Lines [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 956,862,000                  
Net outstanding liabilities 1,041,475,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 90,416,000                  
Standard Personal Lines [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 615,083,000                  
Net outstanding liabilities 647,592,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 38,978,000                  
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 101,134,000 101,099,000 100,667,000 100,539,000 99,454,000 97,335,000 92,019,000 82,455,000 66,884,000 45,846,000
Net outstanding liabilities 101,714,000 101,750,000 101,747,000 101,814,000 101,654,000 101,744,000 102,325,000 103,283,000 102,932,000 97,161,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 254,000                  
Short-duration Insurance Contracts, Number of Reported Claims 15,354                  
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 33,951,000 33,953,000 33,869,000 33,857,000 33,862,000 33,081,000 32,228,000 31,846,000 30,104,000 20,840,000
Net outstanding liabilities 34,010,000 34,025,000 34,056,000 34,566,000 34,422,000 34,186,000 34,273,000 34,926,000 36,547,000 $ 38,589,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 58,000                  
Short-duration Insurance Contracts, Number of Reported Claims 4,570                  
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 $ 98,668,000 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 99,116,000 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 $ 264,000                  
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 $ 37,930,000 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 38,045,000 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 $ 65,000                  
Short-duration Insurance Contracts, Number of Reported Claims 5,139                  
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 $ 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 103,412,000 103,393,000 103,866,000 104,734,000 103,908,000 105,033,000 103,319,000 93,808,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 256,000                  
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 $ 40,189,000 39,907,000 39,819,000 39,731,000 39,342,000 38,078,000 36,965,000 28,299,000    
Net outstanding liabilities 40,457,000 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 $ 74,000                  
Short-duration Insurance Contracts, Number of Reported Claims 5,631                  
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 $ 106,453,000 105,849,000 104,061,000 101,540,000 95,300,000 82,490,000 58,786,000      
Net outstanding liabilities 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 $ 277,000                  
Short-duration Insurance Contracts, Number of Reported Claims 20,821                  
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 $ 62,241,000 62,227,000 62,155,000 61,106,000 60,295,000 58,638,000 43,699,000      
Net outstanding liabilities 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 $ 86,000                  
Short-duration Insurance Contracts, Number of Reported Claims 9,128                  
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 $ 112,732,000 111,085,000 105,068,000 93,878,000 82,102,000 61,323,000        
Net outstanding liabilities 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 $ 644,000                  
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 $ 93,720,000 93,312,000 92,185,000 91,718,000 89,963,000 71,668,000        
Net outstanding liabilities 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 $ 143,000                  
Short-duration Insurance Contracts, Number of Reported Claims 15,102                  
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 $ 107,890,000 102,977,000 94,842,000 82,729,000 63,704,000          
Net outstanding liabilities 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 $ 988,000                  
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 $ 85,196,000 84,250,000 82,720,000 79,584,000 69,056,000          
Net outstanding liabilities 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 $ 251,000                  
Short-duration Insurance Contracts, Number of Reported Claims 16,927                  
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 $ 100,528,000 92,637,000 80,861,000 61,384,000            
Net outstanding liabilities 106,703,000 106,225,000 109,620,000 108,417,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,252,000                  
Short-duration Insurance Contracts, Number of Reported Claims 22,359                  
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 $ 69,775,000 67,838,000 65,528,000 50,664,000            
Net outstanding liabilities 72,145,000 71,494,000 72,528,000 73,670,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,545,000                  
Short-duration Insurance Contracts, Number of Reported Claims 7,738                  
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 $ 92,589,000 83,739,000 62,519,000              
Net outstanding liabilities 106,757,000 109,325,000 102,250,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 6,945,000                  
Short-duration Insurance Contracts, Number of Reported Claims 22,478                  
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 $ 79,751,000 76,007,000 61,561,000              
Net outstanding liabilities 83,637,000 82,461,000 80,111,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,928,000                  
Short-duration Insurance Contracts, Number of Reported Claims 8,739                  
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 $ 76,470,000 58,725,000                
Net outstanding liabilities 99,698,000 96,387,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 13,594,000                  
Short-duration Insurance Contracts, Number of Reported Claims 20,797                  
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 $ 70,078,000 52,589,000                
Net outstanding liabilities 76,400,000 76,637,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,984,000                  
Short-duration Insurance Contracts, Number of Reported Claims 7,677                  
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 $ 57,961,000                  
Net outstanding liabilities 92,727,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 18,187,000                  
Short-duration Insurance Contracts, Number of Reported Claims 19,044                  
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 $ 42,252,000                  
Net outstanding liabilities 60,105,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 5,646,000                  
Short-duration Insurance Contracts, Number of Reported Claims 6,402                  
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net $ 5,803,000                  
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 6,469,000                  
E&S Lines [Member]                    
Claims Development [Line Items]                    
Reserve for losses and loss expenses           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 125,117,000                  
Net outstanding liabilities 368,730,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 243,613,000                  
E&S Lines [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 13,251,000                  
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 0 0 0 0 0 0        
Net outstanding liabilities 0 0 0 0 0 0        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 0                  
Short-duration Insurance Contracts, Number of Reported Claims 0                  
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 $ 92,000 79,000 80,000 70,000 24,000 0        
Net outstanding liabilities (162,000) 52,000 119,000 146,000 169,000 92,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (270,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 35                  
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 $ 709,000 626,000 605,000 431,000 198,000 0        
Net outstanding liabilities 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 $ (630,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 274                  
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 $ 4,513,000 4,078,000 3,574,000 2,570,000 1,218,000 0        
Net outstanding liabilities 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 $ (1,778,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 797                  
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 $ 9,912,000 9,954,000 6,445,000 3,200,000 806,000 0        
Net outstanding liabilities 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 $ (599,000)                  
Short-duration Insurance Contracts, Number of Reported Claims 1,303                  
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 $ 32,343,000 25,064,000 16,430,000 7,914,000 3,722,000          
Net outstanding liabilities 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 $ 9,289,000                  
Short-duration Insurance Contracts, Number of Reported Claims 1,982                  
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 $ 35,200,000 21,980,000 9,470,000 2,715,000            
Net outstanding liabilities 69,112,000 67,099,000 60,309,000 $ 55,468,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 21,956,000                  
Short-duration Insurance Contracts, Number of Reported Claims 2,128                  
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 $ 25,571,000 12,234,000 2,353,000              
Net outstanding liabilities 69,929,000 63,505,000 $ 55,316,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 29,236,000                  
Short-duration Insurance Contracts, Number of Reported Claims 1,888                  
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 $ 13,057,000 3,036,000                
Net outstanding liabilities 76,432,000 $ 75,498,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 48,390,000                  
Short-duration Insurance Contracts, Number of Reported Claims 2,313                  
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 $ 3,720,000                  
Net outstanding liabilities 94,451,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 84,328,000                  
Short-duration Insurance Contracts, Number of Reported Claims 1,760                  
E&S Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2007 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net $ 0