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Reserves For Losses And Loss Expenses (Loss and Loss Expenses Incurred, Net of Reinsurance, and Cumulative Paid Claims and Claim Adjustment Expenses, Net of Reinsurance) (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
claim
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Dec. 31, 2009
USD ($)
Claims Development [Line Items]                    
Reserves for loss and loss adjustment expenses, net of reinsurance $ 10,811,975                  
Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 8,326,367                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,751,400                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 121,405                  
Reserves for loss and loss adjustment expenses, net of reinsurance 3,696,372                  
Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 5,801,492                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,061,332                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 170,897                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,911,057                  
Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 721,272                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 141,892                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 740,877                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,320,257                  
Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 2,586,052                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,685,460                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 4,198                  
Reserves for loss and loss adjustment expenses, net of reinsurance 904,790                  
Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 3,901,666                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 3,270,276                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 2,482                  
Reserves for loss and loss adjustment expenses, net of reinsurance 633,872                  
Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 5,836,156                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,307,301                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 2,362                  
Reserves for loss and loss adjustment expenses, net of reinsurance 531,217                  
Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 2,721,144                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,577,946                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 369,801                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,512,999                  
Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,088,864                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 900,008                  
Reserves for loss and loss adjustment expenses before 2009, net of reinsurance 776                  
Reserves for loss and loss adjustment expenses, net of reinsurance 189,632                  
Accident Year 2009 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 542,568 $ 546,746 $ 553,058 $ 557,661 $ 561,784 $ 589,659 $ 599,235 $ 624,991 $ 656,788 $ 689,634
Cumulative paid claims and claim adjustment expenses, net of reinsurance 510,474 500,953 486,893 470,830 429,128 385,068 311,979 214,498 122,850 44,801
IBNR $ 18,690                  
Cumulative Number of Reported Claims | claim 23,000                  
Accident Year 2009 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 401,431 398,994 395,288 392,287 394,303 392,158 386,870 326,766 332,303 327,537
Cumulative paid claims and claim adjustment expenses, net of reinsurance 364,712 360,289 352,516 344,771 333,793 317,796 297,079 257,972 197,736 93,647
IBNR $ 10,693                  
Cumulative Number of Reported Claims | claim 43,000                  
Accident Year 2009 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 130,790 129,249 134,716 142,768 138,765 148,189 148,223 153,912 153,766 168,762
Cumulative paid claims and claim adjustment expenses, net of reinsurance 41,243 37,817 33,573 29,150 25,272 20,729 14,138 11,037 8,402 5,060
IBNR $ 21,734                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2009 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 157,451 152,711 153,492 150,783 150,452 147,994 148,899 145,515 139,091 134,784
Cumulative paid claims and claim adjustment expenses, net of reinsurance 151,499 147,599 144,713 143,950 138,876 127,728 117,683 85,902 52,583 12,604
IBNR $ 1,087                  
Cumulative Number of Reported Claims | claim 3,000                  
Accident Year 2009 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 334,926 334,977 335,089 334,652 336,855 337,915 335,851 340,967 345,139 362,302
Cumulative paid claims and claim adjustment expenses, net of reinsurance 333,677 333,605 333,143 331,482 328,843 312,902 291,925 257,326 209,553 136,433
IBNR $ 398                  
Cumulative Number of Reported Claims | claim 39,000                  
Accident Year 2009 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 317,098 316,220 314,010 314,117 314,290 318,349 318,111 326,440 335,921 346,870
Cumulative paid claims and claim adjustment expenses, net of reinsurance 315,966 311,655 311,357 311,079 310,428 309,916 306,020 304,634 291,338 212,521
IBNR $ 550                  
Cumulative Number of Reported Claims | claim 18,000                  
Accident Year 2009 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 279,372 281,592 288,115 282,411 293,275 301,495 309,228 327,236 328,321 334,804
Cumulative paid claims and claim adjustment expenses, net of reinsurance 228,990 221,141 211,143 196,775 181,946 155,124 124,030 85,761 53,636 21,332
IBNR 17,086                  
Accident Year 2009 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 35,763 35,293 36,263 36,913 37,505 38,124 38,711 42,352 43,193 48,029
Cumulative paid claims and claim adjustment expenses, net of reinsurance 35,158 34,027 34,759 32,540 31,238 31,248 29,444 28,226 21,945 $ 9,766
IBNR 0                  
Accident Year 2010 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 566,695 571,305 573,532 574,780 577,679 590,818 592,117 615,797 612,210  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 525,016 508,359 491,104 461,442 417,166 336,243 248,698 128,946 45,193  
IBNR $ 26,658                  
Cumulative Number of Reported Claims | claim 23,000                  
Accident Year 2010 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 424,374 427,698 429,762 429,952 426,622 418,315 409,237 361,808 358,734  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 388,405 382,665 374,013 362,078 344,631 317,986 279,226 214,034 107,742  
IBNR $ 13,958                  
Cumulative Number of Reported Claims | claim 45,000                  
Accident Year 2010 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 95,374 94,986 100,065 104,732 100,331 118,712 120,272 123,497 135,639  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 22,355 18,821 14,261 11,699 9,084 8,701 5,571 4,003 2,867  
IBNR $ 14,497                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2010 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 182,818 177,844 174,883 172,585 176,723 177,957 179,383 165,755 147,649  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 178,879 171,330 165,018 160,598 144,474 129,757 108,566 58,916 14,832  
IBNR $ 1,807                  
Cumulative Number of Reported Claims | claim 4,000                  
Accident Year 2010 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 329,942 330,263 330,552 331,581 332,716 328,854 330,190 320,098 310,591  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 327,829 327,206 326,770 324,963 313,253 295,347 263,639 208,790 136,029  
IBNR $ 372                  
Cumulative Number of Reported Claims | claim 37,000                  
Accident Year 2010 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 346,172 346,463 346,885 346,719 346,226 355,916 358,373 370,291 385,541  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 345,106 345,085 344,102 342,918 340,210 346,768 337,686 325,156 245,036  
IBNR $ 833                  
Cumulative Number of Reported Claims | claim 19,000                  
Accident Year 2010 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 248,145 249,062 251,308 254,550 265,771 276,234 288,141 298,300 290,285  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 190,295 180,544 165,027 149,322 129,041 106,381 77,191 45,626 17,964  
IBNR 18,428                  
Accident Year 2010 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 50,932 50,699 50,871 50,971 51,500 51,448 52,561 55,647 58,576  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 50,053 49,303 49,048 46,419 44,824 43,898 42,413 37,739 $ 23,654  
IBNR 201                  
Accident Year 2011 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 634,264 645,123 649,055 653,864 659,026 660,023 673,730 665,035    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 575,194 556,110 524,314 470,849 379,845 266,780 142,713 48,825    
IBNR $ 30,381                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2011 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 471,471 475,729 474,076 472,087 470,026 454,797 442,550 419,364    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 428,811 420,945 408,304 385,759 355,909 307,873 234,694 106,157    
IBNR $ 17,942                  
Cumulative Number of Reported Claims | claim 46,000                  
Accident Year 2011 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 74,109 78,246 83,850 85,299 87,064 98,051 93,993 88,650    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 21,463 20,164 18,638 15,684 12,104 6,395 4,848 2,593    
IBNR $ 17,280                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2011 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 175,963 176,865 173,545 177,128 190,096 186,918 165,233 179,875    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 168,874 167,286 159,014 150,840 134,608 103,097 62,442 18,779    
IBNR $ 3,185                  
Cumulative Number of Reported Claims | claim 4,000                  
Accident Year 2011 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 343,524 342,052 341,162 341,362 331,694 328,269 320,898 312,224    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 340,319 338,283 333,949 321,786 296,332 262,659 211,756 135,350    
IBNR $ 1,232                  
Cumulative Number of Reported Claims | claim 37,000                  
Accident Year 2011 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 451,211 449,320 450,115 457,011 459,814 463,006 471,555 478,556    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 450,155 447,342 446,745 445,073 440,777 436,585 417,701 303,012    
IBNR $ 1,093                  
Cumulative Number of Reported Claims | claim 21,000                  
Accident Year 2011 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 292,162 295,576 303,867 306,911 299,628 304,409 309,621 290,635    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 220,011 207,692 191,864 168,244 134,285 97,702 52,365 17,876    
IBNR 21,264                  
Accident Year 2011 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 84,882 84,471 84,739 85,006 86,544 85,118 87,970 95,217    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 83,401 82,322 81,780 78,577 76,010 73,359 58,875 $ 31,478    
IBNR 314                  
Accident Year 2012 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 718,166 723,610 713,266 709,026 703,118 702,342 693,447      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 622,687 580,750 513,644 417,686 299,842 158,869 58,108      
IBNR $ 47,001                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2012 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 508,169 512,707 509,167 503,863 503,956 499,882 499,752      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 451,991 437,196 419,588 387,368 339,560 253,781 114,998      
IBNR $ 26,626                  
Cumulative Number of Reported Claims | claim 48,000                  
Accident Year 2012 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 69,361 67,878 72,441 73,653 71,780 73,230 72,366      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 17,982 15,738 13,234 11,167 9,480 6,097 1,127      
IBNR $ 16,293                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2012 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 243,893 245,076 238,704 250,457 264,808 241,944 238,233      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 223,424 214,315 190,295 159,183 128,281 87,008 21,875      
IBNR $ 9,507                  
Cumulative Number of Reported Claims | claim 7,000                  
Accident Year 2012 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 366,662 364,437 364,175 355,433 342,379 326,585 314,073      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 360,799 355,740 344,428 312,178 273,277 215,078 136,844      
IBNR $ 1,539                  
Cumulative Number of Reported Claims | claim 40,000                  
Accident Year 2012 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 508,565 506,705 507,646 531,729 535,885 535,500 526,312      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 503,738 499,446 498,506 513,733 503,364 453,157 281,456      
IBNR $ 3,083                  
Cumulative Number of Reported Claims | claim 25,000                  
Accident Year 2012 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 333,889 335,636 332,932 324,014 331,006 335,661 331,603      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 240,926 219,106 186,483 151,635 111,928 62,198 22,390      
IBNR 25,536                  
Accident Year 2012 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 84,875 83,938 83,925 85,451 86,426 94,720 103,744      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 81,808 79,099 77,540 70,672 64,238 51,774 $ 15,675      
IBNR 626                  
Accident Year 2013 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 811,592 805,288 784,342 784,906 792,464 751,544        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 650,428 589,564 473,933 332,871 189,936 63,868        
IBNR $ 78,784                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2013 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 542,274 547,000 543,238 546,995 547,295 551,342        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 466,580 447,894 414,160 363,028 277,538 117,502        
IBNR $ 35,177                  
Cumulative Number of Reported Claims | claim 53,000                  
Accident Year 2013 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 35,120 38,551 42,419 46,025 48,493 63,995        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,418 3,396 3,008 2,158 630 647        
IBNR $ 16,473                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2013 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 279,092 270,449 248,974 242,792 247,320 269,280        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 248,520 206,655 177,343 119,552 64,030 24,232        
IBNR $ 15,395                  
Cumulative Number of Reported Claims | claim 7,000                  
Accident Year 2013 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 366,565 366,976 376,569 368,318 348,513 326,789        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 353,159 343,274 322,141 266,694 218,005 142,480        
IBNR $ 3,013                  
Cumulative Number of Reported Claims | claim 42,000                  
Accident Year 2013 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 548,730 552,137 553,621 575,582 583,603 572,103        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 539,443 538,158 531,386 534,939 486,692 312,945        
IBNR $ 5,740                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2013 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 298,061 292,447 283,580 273,528 270,221 319,159        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 204,820 177,101 143,268 109,202 63,849 28,920        
IBNR 33,269                  
Accident Year 2013 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 111,876 112,555 111,915 114,063 112,590 141,298        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 106,051 104,333 101,553 92,646 74,602 $ 36,609        
IBNR 1,192                  
Accident Year 2014 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 864,157 851,503 847,008 848,947 847,207          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 595,024 481,039 338,961 191,394 79,078          
IBNR $ 128,856                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2014 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 615,435 617,242 627,767 637,307 639,436          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 503,915 471,235 412,611 319,743 148,405          
IBNR $ 57,455                  
Cumulative Number of Reported Claims | claim 57,000                  
Accident Year 2014 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 41,671 45,758 49,478 57,558 63,465          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,808 4,175 3,354 1,729 358          
IBNR $ 18,448                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2014 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 239,555 243,936 259,964 246,668 253,284          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 199,245 176,181 138,753 83,856 19,545          
IBNR $ 19,315                  
Cumulative Number of Reported Claims | claim 8,000                  
Accident Year 2014 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 413,697 416,194 418,215 384,692 363,308          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 394,147 365,424 328,156 237,118 155,065          
IBNR $ 6,431                  
Cumulative Number of Reported Claims | claim 45,000                  
Accident Year 2014 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 664,350 663,282 664,303 709,832 701,335          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 648,373 633,020 613,621 596,829 371,194          
IBNR $ 6,562                  
Cumulative Number of Reported Claims | claim 30,000                  
Accident Year 2014 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 326,251 331,836 319,855 320,176 320,250          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 198,993 155,764 116,266 69,134 21,306          
IBNR 49,301                  
Accident Year 2014 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 99,410 100,148 97,363 96,668 112,987          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 91,646 88,507 82,329 67,000 $ 38,919          
IBNR 1,739                  
Accident Year 2015 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 964,415 961,289 986,792 951,041            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 538,502 382,498 210,940 82,829            
IBNR $ 217,570                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2015 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 641,169 650,997 690,525 712,800            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 477,541 421,734 323,744 139,320            
IBNR $ 83,941                  
Cumulative Number of Reported Claims | claim 58,000                  
Accident Year 2015 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 45,115 50,099 57,897 69,977            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,099 3,272 2,481 2,069            
IBNR $ 25,425                  
Cumulative Number of Reported Claims | claim 0                  
Accident Year 2015 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 276,406 274,950 258,251 259,569            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 187,767 139,952 85,561 20,478            
IBNR $ 47,934                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2015 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 431,857 423,601 417,403 389,101            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 370,450 325,369 265,396 159,679            
IBNR $ 18,247                  
Cumulative Number of Reported Claims | claim 50,000                  
Accident Year 2015 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 726,606 727,677 731,468 743,872            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 689,875 667,846 612,335 396,086            
IBNR $ 15,079                  
Cumulative Number of Reported Claims | claim 32,000                  
Accident Year 2015 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 253,264 231,020 232,203 259,609            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 141,834 91,548 48,593 17,865            
IBNR 30,511                  
Accident Year 2015 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 130,452 131,777 117,582 127,039            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 118,686 109,187 89,228 $ 53,498            
IBNR 4,367                  
Accident Year 2016 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,019,749 1,011,368 1,018,454              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 390,552 209,212 69,620              
IBNR $ 375,609                  
Cumulative Number of Reported Claims | claim 25,000                  
Accident Year 2016 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 684,700 696,339 702,716              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 446,072 338,835 142,998              
IBNR $ 117,425                  
Cumulative Number of Reported Claims | claim 57,000                  
Accident Year 2016 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 71,404 70,281 72,657              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,573 4,783 2,498              
IBNR $ 37,500                  
Cumulative Number of Reported Claims | claim 0                  
Accident Year 2016 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 361,929 324,979 310,678              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 202,131 102,853 28,702              
IBNR $ 90,872                  
Cumulative Number of Reported Claims | claim 10,000                  
Accident Year 2016 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 443,030 431,680 431,633              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 342,214 280,146 185,045              
IBNR $ 35,867                  
Cumulative Number of Reported Claims | claim 49,000                  
Accident Year 2016 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 761,891 774,514 771,416              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 710,618 669,029 416,144              
IBNR $ 20,857                  
Cumulative Number of Reported Claims | claim 34,000                  
Accident Year 2016 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 246,235 253,450 241,282              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 100,587 61,930 19,923              
IBNR 62,140                  
Accident Year 2016 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 181,858 174,562 167,989              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 157,622 133,653 $ 78,969              
IBNR 11,130                  
Accident Year 2017 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,100,243 1,067,376                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 256,448 80,174                
IBNR $ 567,982                  
Cumulative Number of Reported Claims | claim 23,000                  
Accident Year 2017 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 733,505 762,093                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 362,299 153,456                
IBNR $ 191,034                  
Cumulative Number of Reported Claims | claim 57,000                  
Accident Year 2017 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 80,508 76,701                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 12,810 6,282                
IBNR $ 42,652                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2017 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 333,194 333,803                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 96,814 36,733                
IBNR $ 178,091                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2017 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 428,601 430,352                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 267,469 180,627                
IBNR $ 70,289                  
Cumulative Number of Reported Claims | claim 44,000                  
Accident Year 2017 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 752,193 750,786                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 688,821 444,407                
IBNR $ 34,275                  
Cumulative Number of Reported Claims | claim 38,000                  
Accident Year 2017 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 221,820 231,826                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 40,338 16,493                
IBNR 122,752                  
Accident Year 2017 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 200,535 206,604                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 141,458 $ 72,157                
IBNR 24,102                  
Accident Year 2018 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,104,518                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 87,075                  
IBNR $ 837,548                  
Cumulative Number of Reported Claims | claim 18,000                  
Accident Year 2018 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 778,964                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 171,006                  
IBNR $ 359,337                  
Cumulative Number of Reported Claims | claim 53,000                  
Accident Year 2018 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 77,820                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,141                  
IBNR $ 46,840                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2018 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 335,751                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 28,307                  
IBNR $ 260,095                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2018 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 442,862                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 180,213                  
IBNR $ 145,744                  
Cumulative Number of Reported Claims | claim 39,000                  
Accident Year 2018 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 759,340                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 415,206                  
IBNR $ 161,107                  
Cumulative Number of Reported Claims | claim 36,000                  
Accident Year 2018 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 221,945                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 11,152                  
IBNR 184,317                  
Accident Year 2018 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 108,281                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 34,125                  
IBNR $ 40,135