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Reserve for Loss and Loss Expense (Claims Development) (Details)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
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 ($)
Dec. 31, 2010
USD ($)
Claims Development [Line Items]                    
Net outstanding liabilities $ 11,547,343,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 8,514,750,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 3,392,713,000                  
Reserve for loss and loss expense 4,067,163,000 $ 3,893,868,000                
Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 360,119,000                  
Short-Duration Insurance Contracts, Accident Years 2010 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 902,258,000 904,561,000 $ 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 $ 38,152,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 94,526                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 851,633,000 846,386,000 835,532,000 823,770,000 803,773,000 773,536,000 704,895,000 625,229,000 509,910,000 328,826,000
Short-duration Insurance Contracts, Accident Year 2011 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,013,175,000 1,013,115,000 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 $ 44,453,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 104,861                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 950,836,000 940,626,000 924,111,000 901,801,000 852,202,000 782,655,000 692,730,000 585,867,000 391,944,000  
Short-duration Insurance Contracts, Accident Year 2012 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 968,536,000 973,411,000 973,644,000 973,089,000 998,028,000 1,020,655,000 1,071,290,000 1,065,437,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 50,942,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 104,148                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 898,269,000 879,372,000 856,195,000 810,135,000 743,742,000 651,544,000 555,819,000 378,067,000    
Short-duration Insurance Contracts, Accident Year 2013 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 984,858,000 987,763,000 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 $ 72,970,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 91,326                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 891,841,000 872,331,000 833,823,000 748,758,000 644,475,000 518,872,000 335,956,000      
Short-duration Insurance Contracts, Accident Year 2014 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,100,208,000 1,104,218,000 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 $ 83,392,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 95,081                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 981,868,000 936,425,000 855,959,000 736,154,000 614,075,000 405,898,000        
Short-duration Insurance Contracts, Accident Year 2015 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,119,441,000 1,138,313,000 1,144,830,000 1,130,513,000 1,114,081,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 111,657,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 94,128                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 929,222,000 845,868,000 725,385,000 581,203,000 376,641,000          
Short-duration Insurance Contracts, Accident Year 2016 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,199,734,000 1,227,142,000 1,203,634,000 1,188,608,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 205,126,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 94,579                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 892,390,000 764,331,000 617,958,000 387,272,000            
Short-duration Insurance Contracts, Accident Year 2017 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,313,585,000 1,313,372,000 1,270,110,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 336,155,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 98,014                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 829,134,000 678,453,000 433,440,000              
Short-duration Insurance Contracts, Accident Year 2018 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,461,603,000 1,413,800,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 501,519,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 104,187                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 779,466,000 511,271,000                
Short-Duration Insurance Contract, Accident Year 2019 [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,483,945,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 759,853,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 93,947                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 510,091,000                  
Standard Commercial Lines [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 2,508,023,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,321,589,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 1,280,416,000                  
Standard Commercial Lines [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,789,692,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,179,326,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 850,505,000                  
Standard Commercial Lines [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 2,566,548,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,973,765,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 596,686,000                  
Standard Commercial Lines [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 512,159,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 432,942,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 86,747,000                  
Standard Commercial Lines [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,320,677,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 1,268,844,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 52,071,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 93,982,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 240,140,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 3,904,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 7,530,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 237,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 205,322,000 206,146,000 202,394,000 208,968,000 211,619,000 222,328,000 237,154,000 242,499,000 228,680,000 215,208,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 17,542,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 12,705                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 185,962,000 183,988,000 178,179,000 172,394,000 161,487,000 143,360,000 113,050,000 80,018,000 46,201,000 15,726,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 197,095,000 199,539,000 204,423,000 208,155,000 212,448,000 214,916,000 211,030,000 212,815,000 214,469,000 198,371,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 19,514,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 12,192                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 167,478,000 165,526,000 162,796,000 158,078,000 153,795,000 149,086,000 137,184,000 122,442,000 93,281,000 46,795,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 172,995,000 173,080,000 173,471,000 173,157,000 172,969,000 179,854,000 181,923,000 187,778,000 189,305,000 187,562,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 682,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 25,512                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 171,941,000 171,693,000 169,793,000 169,100,000 167,227,000 163,513,000 146,913,000 128,015,000 99,254,000 68,098,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 41,156,000 40,920,000 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 $ 333,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 3,920                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 40,823,000 40,439,000 40,395,000 40,279,000 38,900,000 37,819,000 34,705,000 31,027,000 28,131,000 20,821,000
Standard Commercial Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 95,338,000 95,142,000 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 $ 4,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 7,669                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 95,334,000 95,138,000 95,150,000 95,156,000 95,147,000 95,270,000 95,111,000 94,602,000 91,918,000 69,543,000
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 213,485,000 211,500,000 212,011,000 211,196,000 217,256,000 230,785,000 239,480,000 228,720,000 227,769,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 19,913,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 11,649                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 187,276,000 181,856,000 170,525,000 159,768,000 135,377,000 102,978,000 73,643,000 42,692,000 13,924,000  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 192,863,000 194,821,000 200,674,000 205,708,000 210,591,000 215,114,000 214,743,000 218,973,000 205,238,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 22,717,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 11,860                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 161,696,000 158,535,000 154,320,000 149,269,000 139,232,000 134,646,000 118,847,000 90,836,000 42,941,000  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 173,401,000 173,507,000 174,514,000 174,882,000 172,617,000 178,421,000 182,325,000 183,044,000 174,006,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 899,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 25,524                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 172,365,000 170,913,000 170,000,000 166,082,000 157,291,000 142,507,000 121,576,000 99,196,000 69,849,000  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 58,948,000 58,735,000 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 $ 362,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 4,960                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 57,380,000 57,365,000 57,045,000 55,856,000 52,114,000 46,444,000 41,011,000 37,362,000 27,884,000  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 131,002,000 131,009,000 131,049,000 131,113,000 131,353,000 131,282,000 130,942,000 131,667,000 136,954,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 7,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 9,038                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 131,092,000 131,100,000 131,089,000 131,115,000 131,060,000 130,681,000 129,579,000 127,580,000 94,538,000  
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 182,085,000 180,659,000 175,268,000 175,305,000 194,144,000 215,083,000 245,561,000 238,979,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 21,005,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 9,994                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 156,186,000 144,451,000 130,866,000 109,448,000 89,008,000 56,580,000 35,241,000 13,030,000    
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 178,774,000 183,314,000 187,359,000 188,596,000 195,197,000 199,360,000 208,036,000 203,864,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 24,605,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 11,618                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 146,739,000 143,281,000 139,477,000 132,052,000 122,755,000 108,211,000 86,909,000 40,911,000    
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 185,357,000 184,633,000 186,128,000 184,367,000 184,289,000 183,527,000 191,947,000 179,551,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,920,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 24,160                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 181,353,000 179,501,000 176,656,000 168,114,000 148,669,000 127,235,000 105,371,000 73,316,000    
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 43,717,000 43,418,000 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 $ 703,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 5,543                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 41,356,000 41,326,000 40,627,000 38,766,000 37,215,000 35,089,000 31,833,000 22,199,000    
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 117,220,000 117,225,000 117,170,000 117,102,000 116,658,000 115,375,000 114,224,000 118,464,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 10,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 8,517                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 116,674,000 116,671,000 116,625,000 116,291,000 114,699,000 111,503,000 108,834,000 81,528,000    
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 202,697,000 203,831,000 210,785,000 225,709,000 239,776,000 251,421,000 250,609,000      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 28,857,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,378                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 163,764,000 153,628,000 139,114,000 104,587,000 72,127,000 35,113,000 12,789,000      
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 159,767,000 162,787,000 166,662,000 173,160,000 187,658,000 194,318,000 199,794,000      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 26,260,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 11,375                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 126,822,000 124,130,000 118,669,000 109,739,000 96,376,000 74,568,000 36,829,000      
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 209,602,000 207,975,000 210,410,000 207,994,000 209,197,000 205,282,000 188,289,000      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,928,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 25,722                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 202,622,000 200,750,000 189,626,000 169,850,000 140,015,000 109,893,000 76,469,000      
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 39,699,000 39,709,000 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 $ 803,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 3,482                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 39,085,000 38,464,000 37,993,000 34,760,000 30,845,000 26,592,000 17,412,000      
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 90,218,000 90,278,000 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 $ 18,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 5,713                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 90,646,000 90,696,000 90,840,000 90,350,000 90,446,000 87,874,000 60,244,000      
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 237,125,000 234,082,000 239,333,000 257,132,000 249,946,000 244,312,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 42,388,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,586                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 179,192,000 154,957,000 121,969,000 79,972,000 46,825,000 14,901,000        
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 160,646,000 164,420,000 172,515,000 182,579,000 187,065,000 199,346,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 28,320,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,495                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 124,077,000 119,392,000 113,626,000 100,876,000 78,944,000 35,924,000        
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 217,334,000 218,172,000 219,925,000 216,824,000 212,725,000 200,534,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,831,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 27,714                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 209,655,000 202,821,000 180,701,000 148,884,000 117,169,000 80,810,000        
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 58,093,000 58,517,000 59,806,000 62,548,000 60,949,000 55,962,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,390,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 4,064                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 55,458,000 52,940,000 49,460,000 44,911,000 40,584,000 28,914,000        
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 136,212,000 138,155,000 138,751,000 136,820,000 136,249,000 141,192,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 33,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,515                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 136,008,000 137,418,000 137,883,000 136,634,000 132,909,000 101,131,000        
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 219,204,000 233,249,000 246,990,000 245,710,000 254,720,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 55,244,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,381                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 144,216,000 116,804,000 78,668,000 39,978,000 14,665,000          
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 176,242,000 179,642,000 183,604,000 194,639,000 193,729,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 27,927,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,549                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 120,097,000 112,601,000 98,195,000 77,320,000 33,857,000          
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 260,565,000 259,495,000 253,074,000 240,958,000 220,994,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 7,966,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 29,340                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 238,142,000 211,515,000 175,866,000 132,260,000 91,347,000          
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 54,454,000 55,925,000 57,245,000 53,768,000 52,871,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 3,235,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 3,959                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 49,073,000 46,571,000 42,710,000 36,014,000 24,189,000          
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 112,496,000 111,566,000 111,750,000 109,513,000 110,270,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 56,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,404                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 110,969,000 110,994,000 109,829,000 106,182,000 79,048,000          
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 263,245,000 277,986,000 272,048,000 277,214,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 98,385,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,526                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 133,757,000 89,431,000 46,549,000 15,684,000            
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 166,009,000 176,248,000 184,946,000 196,774,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 41,146,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,572                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 109,166,000 98,037,000 78,531,000 34,525,000            
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 285,304,000 285,302,000 274,367,000 255,187,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 21,279,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 31,167                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 233,939,000 200,701,000 155,720,000 106,022,000            
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 53,835,000 54,993,000 53,792,000 52,335,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 4,245,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 3,843                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 45,308,000 39,973,000 36,848,000 24,655,000            
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 124,487,000 125,937,000 126,185,000 121,927,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (96,000)                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,739                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 123,828,000 122,930,000 118,789,000 83,966,000            
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 301,384,000 293,128,000 293,747,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 161,114,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,706                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 92,355,000 49,470,000 17,366,000              
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 175,853,000 184,306,000 195,202,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 53,654,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,793                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 100,645,000 82,216,000 40,375,000              
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 324,291,000 329,389,000 301,274,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 57,165,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 32,474                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 220,422,000 178,823,000 117,287,000              
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 51,524,000 48,698,000 46,624,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 10,252,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 3,864                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 36,950,000 31,337,000 21,865,000              
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 149,044,000 149,106,000 138,773,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (884,000)                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,886                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 148,589,000 142,338,000 99,047,000              
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 336,326,000 317,934,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 223,228,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 10,656                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 60,784,000 19,531,000                
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 193,818,000 193,894,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 74,399,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 11,078                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 84,780,000 41,122,000                
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 352,487,000 347,908,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 109,922,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 35,034                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 193,788,000 134,867,000                
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 57,202,000 55,024,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 12,432,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 4,159                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 39,791,000 29,995,000                
Standard Commercial Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 190,834,000 183,177,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ (329,000)                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 8,240                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 184,813,000 135,416,000                
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 347,150,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 296,257,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 8,626                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 18,097,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Workers Compensation [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 188,625,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 100,336,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 9,805                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 37,826,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Commercial Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 385,212,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 183,477,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 33,438                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 149,538,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Business Owners' Policies [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 53,531,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 15,068,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 3,210                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 27,718,000                  
Standard Commercial Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Commercial Property [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 173,826,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 15,732,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,722                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 130,891,000                  
Standard Personal Lines [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 1,076,685,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 983,814,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 100,331,000                  
Standard Personal Lines [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 739,559,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 713,124,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 31,752,000                  
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 7,462,000                  
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 5,316,000                  
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 106,785,000 106,887,000 107,054,000 107,224,000 107,405,000 107,490,000 109,515,000 112,346,000 110,075,000 103,340,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 64,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 20,823                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 106,716,000 106,722,000 106,733,000 106,453,000 105,849,000 104,061,000 101,540,000 95,300,000 82,490,000 58,786,000
Standard Personal Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 62,380,000 62,402,000 62,392,000 62,339,000 62,402,000 62,462,000 61,927,000 63,285,000 67,525,000 68,373,000
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 48,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 9,132                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 62,329,000 62,283,000 62,272,000 62,241,000 62,227,000 62,155,000 61,106,000 60,295,000 58,638,000 $ 43,699,000
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 114,056,000 113,921,000 113,988,000 113,830,000 114,241,000 112,993,000 113,686,000 116,164,000 113,232,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 121,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 22,700                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 113,856,000 113,664,000 113,551,000 112,732,000 111,085,000 105,068,000 93,878,000 82,102,000 61,323,000  
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 94,572,000 94,587,000 94,378,000 94,183,000 94,543,000 94,167,000 97,761,000 98,211,000 103,804,000  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 82,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 15,111                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 94,458,000 94,412,000 94,007,000 93,720,000 93,312,000 92,185,000 91,718,000 89,963,000 71,668,000  
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 109,701,000 109,795,000 110,300,000 110,294,000 109,324,000 109,832,000 114,921,000 113,771,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 155,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 22,332                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 109,482,000 109,447,000 109,355,000 107,890,000 102,977,000 94,842,000 82,729,000 63,704,000    
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 86,567,000 86,483,000 86,330,000 86,271,000 86,667,000 86,560,000 82,744,000 87,260,000    
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 94,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 16,941                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 85,897,000 85,642,000 85,562,000 85,196,000 84,250,000 82,720,000 79,584,000 69,056,000    
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 107,916,000 107,680,000 107,759,000 106,703,000 106,225,000 109,620,000 108,417,000      
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 348,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 22,375                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 106,876,000 106,679,000 105,131,000 100,528,000 92,637,000 80,861,000 61,384,000      
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 72,318,000 72,148,000 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 $ 420,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 7,749                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 72,433,000 72,197,000 71,776,000 69,775,000 67,838,000 65,528,000 50,664,000      
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 107,104,000 106,821,000 107,452,000 106,757,000 109,325,000 102,250,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 307,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 22,506                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 105,709,000 104,055,000 99,173,000 92,589,000 83,739,000 62,519,000        
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 83,824,000 83,539,000 83,844,000 83,637,000 82,461,000 80,111,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 682,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 8,773                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 82,836,000 82,583,000 81,664,000 79,751,000 76,007,000 61,561,000        
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 98,718,000 99,570,000 100,214,000 99,698,000 96,387,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 742,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 20,863                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 95,997,000 92,102,000 87,163,000 76,470,000 58,725,000          
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 74,978,000 74,723,000 76,559,000 76,400,000 76,637,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 660,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 7,746                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 74,079,000 72,927,000 72,202,000 70,078,000 52,589,000          
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 101,140,000 100,202,000 98,032,000 92,727,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,565,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 19,819                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 94,372,000 86,752,000 76,823,000 57,961,000            
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 61,723,000 62,391,000 60,931,000 60,105,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 1,221,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,885                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 60,082,000 59,546,000 57,333,000 42,252,000            
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 103,653,000 105,139,000 101,880,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 6,342,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 20,725                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 91,479,000 82,730,000 62,854,000              
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 70,365,000 67,978,000 59,167,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,500,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 7,370                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 67,193,000 63,290,000 45,466,000              
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 113,569,000 111,594,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 14,259,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 22,621                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 89,628,000 69,721,000                
Standard Personal Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 68,526,000 62,961,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 2,366,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 7,554                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 64,137,000 49,430,000                
Standard Personal Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Personal Automobile [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 114,043,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 25,832,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 21,988                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 69,699,000                  
Standard Personal Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | Homeowners [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 64,306,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 6,299,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 6,468                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 49,680,000                  
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]                    
Net outstanding liabilities 716,746,000                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net 350,912,000                  
Liability for Unpaid Claims and Claims Adjustment Expense, Net 365,943,000                  
E&S Lines [Member] | Short-Duration Insurance Contracts, Accident Years Prior to 2010 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Liability for Unpaid Claims and Claims Adjustment Expense, Net 109,000                  
E&S Lines [Member] | Short-Duration Insurance Contracts, Accident Years 2010 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 5,534,000 5,168,000 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 Contract, Cumulative Number of Reported Claims 815                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 5,362,000 4,908,000 4,610,000 4,513,000 4,078,000 3,574,000 2,570,000 1,218,000 0  
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 11,554,000 12,119,000 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 $ 177,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,332                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 9,604,000 9,819,000 10,256,000 9,912,000 9,954,000 6,445,000 3,200,000 806,000 $ 0  
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 44,622,000 46,444,000 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 $ 2,474,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,045                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 39,832,000 38,298,000 36,278,000 32,343,000 25,064,000 16,430,000 7,914,000 $ 3,722,000    
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 68,451,000 68,972,000 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 $ 13,816,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,280                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 54,974,000 51,142,000 46,108,000 35,200,000 21,980,000 9,470,000 $ 2,715,000      
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 71,153,000 71,206,000 71,719,000 69,929,000 63,505,000 55,316,000        
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 5,559,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,071                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 60,092,000 53,780,000 43,877,000 25,571,000 12,234,000 $ 2,353,000        
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 90,355,000 90,488,000 82,404,000 76,432,000 75,498,000          
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 15,752,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,799                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 64,529,000 50,712,000 29,389,000 13,057,000 $ 3,036,000          
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 105,120,000 104,655,000 96,416,000 94,451,000            
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 35,987,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,859                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 56,581,000 33,950,000 16,195,000 $ 3,720,000            
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2017 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 99,866,000 95,783,000 91,438,000              
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 47,074,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,614                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 34,179,000 14,672,000 $ 5,057,000              
E&S Lines [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 103,004,000 98,324,000                
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 62,754,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 2,392                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 21,337,000 $ 5,509,000                
E&S Lines [Member] | Short-Duration Insurance Contract, Accident Year 2019 [Member] | General Liability [Member]                    
Claims Development [Line Items]                    
Net outstanding liabilities 117,087,000                  
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net $ 103,146,000                  
Short-duration Insurance Contract, Cumulative Number of Reported Claims 1,679                  
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net $ 4,422,000