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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, 2020
USD ($)
claim
Dec. 31, 2019
USD ($)
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 ($)
Dec. 31, 2011
USD ($)
Claims Development [Line Items]                    
Reserves for loss and loss adjustment expenses, net of reinsurance $ 12,103,860                  
Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 9,907,708                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,588,033                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 117,290                  
Reserves for loss and loss adjustment expenses, net of reinsurance 4,436,965                  
Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 6,226,489                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,602,267                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 219,529                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,843,751                  
Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 3,154,775                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,965,682                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 14,011                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,203,104                  
Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 4,290,565                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 3,459,309                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 4,119                  
Reserves for loss and loss adjustment expenses, net of reinsurance 835,375                  
Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 6,754,943                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,125,297                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 1,571                  
Reserves for loss and loss adjustment expenses, net of reinsurance 631,217                  
Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 2,931,215                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,666,875                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 380,546                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,644,886                  
Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 617,425                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 120,216                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 735,818                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,233,027                  
Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,196,864                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,019,514                  
Reserves for loss and loss adjustment expenses before 2011, net of reinsurance 1,928                  
Reserves for loss and loss adjustment expenses, net of reinsurance 179,278                  
Accident Year 2011 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 644,565 $ 643,446 $ 632,565 $ 643,358 $ 647,241 $ 652,134 $ 657,143 $ 657,819 $ 671,712 $ 665,626
Cumulative paid claims and claim adjustment expenses, net of reinsurance 604,657 592,637 573,364 554,278 522,488 469,123 377,836 265,109 141,216 48,846
IBNR $ 16,298                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2011 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 472,593 473,766 471,471 475,729 474,076 472,087 470,026 457,134 444,887 413,429
Cumulative paid claims and claim adjustment expenses, net of reinsurance 442,163 436,905 428,811 420,945 408,304 385,759 355,909 309,509 236,207 106,899
IBNR $ 14,609                  
Cumulative Number of Reported Claims | claim 46,000                  
Accident Year 2011 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 181,874 176,944 176,396 177,335 174,038 177,748 190,849 187,797 165,844 180,508
Cumulative paid claims and claim adjustment expenses, net of reinsurance 172,294 170,873 169,191 167,595 159,309 151,135 134,862 103,263 62,560 18,803
IBNR $ 5,325                  
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 $ 342,581 346,490 346,619 345,044 344,174 343,421 334,816 329,917 322,516 314,028
Cumulative paid claims and claim adjustment expenses, net of reinsurance 341,781 340,799 340,360 338,325 333,987 321,814 296,370 262,685 211,756 135,350
IBNR $ 100                  
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 $ 456,106 463,053 459,557 459,369 461,629 467,205 467,989 472,041 490,235 494,704
Cumulative paid claims and claim adjustment expenses, net of reinsurance 455,702 454,991 450,432 447,619 447,016 445,323 441,025 436,787 417,792 303,049
IBNR $ 550                  
Cumulative Number of Reported Claims | claim 22,000                  
Accident Year 2011 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 298,292 301,670 295,376 299,076 307,772 309,235 301,847 306,721 312,321 293,274
Cumulative paid claims and claim adjustment expenses, net of reinsurance 246,140 234,552 222,618 210,245 194,320 170,629 134,814 98,086 52,597 17,976
IBNR 13,368                  
Accident Year 2011 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 69,205 72,091 74,109 78,246 83,850 87,030 89,031 98,051 93,993 88,650
Cumulative paid claims and claim adjustment expenses, net of reinsurance 24,842 23,686 21,463 20,164 18,638 15,684 14,042 6,395 4,848 2,593
IBNR 10,955                  
Accident Year 2011 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 84,112 84,938 85,160 84,744 85,028 85,304 86,879 85,462 88,345 95,737
Cumulative paid claims and claim adjustment expenses, net of reinsurance 83,229 83,750 83,669 82,587 82,038 78,835 76,267 73,601 59,054 $ 31,545
IBNR 366                  
Accident Year 2012 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 713,714 715,466 717,453 722,946 712,698 708,805 702,022 701,212 692,549  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 667,749 652,178 621,454 579,454 512,283 416,561 298,644 157,499 57,599  
IBNR $ 23,827                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2012 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 506,827 506,730 508,169 512,707 509,167 503,863 503,956 501,810 501,681  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 466,028 459,119 451,991 437,196 419,588 387,368 339,560 255,063 115,536  
IBNR $ 19,328                  
Cumulative Number of Reported Claims | claim 48,000                  
Accident Year 2012 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 249,846 247,072 246,478 247,789 241,342 253,146 268,761 245,962 242,530  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 236,481 233,637 225,442 216,199 191,702 160,243 129,282 87,951 22,269  
IBNR $ 5,919                  
Cumulative Number of Reported Claims | claim 6,000                  
Accident Year 2012 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 366,649 366,493 367,216 364,966 364,717 355,609 342,588 326,831 314,309  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 363,587 362,426 361,342 356,264 344,952 312,342 273,446 215,214 136,844  
IBNR $ 365                  
Cumulative Number of Reported Claims | claim 41,000                  
Accident Year 2012 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 509,058 508,294 509,216 507,344 508,366 537,142 542,017 540,714 532,843  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 506,308 505,183 504,451 500,181 499,014 519,093 508,333 456,872 282,703  
IBNR $ 2,406                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2012 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 337,437 334,461 337,901 340,026 337,777 326,934 334,299 339,236 335,187  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 279,123 259,429 243,586 221,679 188,955 153,950 112,474 62,563 22,516  
IBNR 14,927                  
Accident Year 2012 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 66,269 67,205 69,361 67,878 72,441 75,274 73,670 73,230 72,366  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 22,528 20,004 17,982 15,738 13,234 11,167 10,815 6,097 1,127  
IBNR 9,184                  
Accident Year 2012 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 91,342 85,025 85,222 84,286 84,285 85,846 86,809 95,164 104,336  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 89,194 83,007 82,129 79,407 77,842 70,955 64,489 51,964 $ 15,725  
IBNR 1,429                  
Accident Year 2013 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 810,500 806,348 811,775 805,447 784,487 784,674 791,768 750,993    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 721,879 695,301 649,657 588,650 472,731 331,779 188,411 63,358    
IBNR $ 41,398                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2013 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 540,322 541,926 542,274 547,000 543,238 546,995 547,295 552,570    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 489,075 479,104 466,580 447,894 414,160 363,028 277,538 117,900    
IBNR $ 23,485                  
Cumulative Number of Reported Claims | claim 53,000                  
Accident Year 2013 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 284,837 287,310 282,150 273,679 252,024 245,984 251,419 275,051    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 265,297 260,023 250,507 208,368 178,579 120,224 64,439 24,893    
IBNR $ 10,867                  
Cumulative Number of Reported Claims | claim 6,000                  
Accident Year 2013 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 365,820 366,885 367,720 368,106 377,696 368,894 349,136 327,514    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 363,805 363,038 354,280 344,357 323,227 267,253 218,596 142,929    
IBNR $ 586                  
Cumulative Number of Reported Claims | claim 44,000                  
Accident Year 2013 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 547,932 548,672 550,604 554,167 555,715 582,666 591,686 580,902    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 543,195 541,990 541,400 540,132 533,269 541,563 491,801 315,304    
IBNR $ 2,160                  
Cumulative Number of Reported Claims | claim 25,000                  
Accident Year 2013 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 305,080 307,426 302,768 297,121 288,556 276,688 273,683 322,718    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 244,082 227,884 207,312 179,494 145,508 111,189 64,239 29,122    
IBNR 21,094                  
Accident Year 2013 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 29,758 31,752 35,120 38,551 42,419 48,143 50,355 63,995    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,476 5,349 4,418 3,396 3,008 2,158 1,897 647    
IBNR 11,097                  
Accident Year 2013 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 108,017 110,165 112,373 113,053 112,419 114,638 113,201 142,340    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 104,863 108,062 106,483 104,756 101,947 92,968 74,806 $ 36,668    
IBNR 1,757                  
Accident Year 2014 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 867,239 872,150 866,096 853,826 849,440 851,404 848,658      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 732,313 681,976 595,794 481,706 339,194 191,320 79,111      
IBNR $ 66,901                  
Cumulative Number of Reported Claims | claim 27,000                  
Accident Year 2014 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 600,194 604,030 615,435 617,242 627,767 637,307 639,436      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 531,475 521,141 503,915 471,235 412,611 319,743 148,405      
IBNR $ 36,001                  
Cumulative Number of Reported Claims | claim 57,000                  
Accident Year 2014 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 259,892 261,150 242,449 246,983 263,540 250,025 257,442      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 229,615 218,055 201,002 177,575 139,934 84,457 19,773      
IBNR $ 19,573                  
Cumulative Number of Reported Claims | claim 7,000                  
Accident Year 2014 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 411,158 415,621 415,727 418,150 420,139 385,329 363,996      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 406,795 404,087 396,089 367,283 329,979 237,765 155,614      
IBNR $ 1,506                  
Cumulative Number of Reported Claims | claim 47,000                  
Accident Year 2014 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 669,729 667,278 667,061 666,246 667,627 719,508 714,360      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 660,640 658,053 650,762 635,415 615,833 604,942 374,757      
IBNR $ 2,930                  
Cumulative Number of Reported Claims | claim 30,000                  
Accident Year 2014 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 339,941 328,289 328,741 334,617 323,130 323,920 323,837      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 254,532 229,562 199,846 156,495 116,908 69,568 21,441      
IBNR 25,778                  
Accident Year 2014 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 42,618 42,541 41,671 45,758 49,478 57,650 63,561      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 11,154 7,595 5,808 4,175 3,354 1,729 377      
IBNR 15,319                  
Accident Year 2014 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 99,924 99,535 99,892 100,640 97,933 97,411 113,962      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 95,222 93,723 92,011 88,857 82,646 67,251 $ 39,019      
IBNR 2,185                  
Accident Year 2015 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 979,014 968,783 966,872 963,451 988,569 953,198        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 758,890 677,386 539,242 382,869 211,212 82,817        
IBNR $ 99,480                  
Cumulative Number of Reported Claims | claim 27,000                  
Accident Year 2015 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 620,741 626,432 641,169 650,997 690,525 712,800        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 531,512 512,933 477,541 421,734 323,744 139,320        
IBNR $ 50,083                  
Cumulative Number of Reported Claims | claim 58,000                  
Accident Year 2015 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 285,557 294,435 278,868 278,012 261,069 262,459        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 234,585 217,625 188,608 140,517 85,945 20,565        
IBNR $ 29,712                  
Cumulative Number of Reported Claims | claim 8,000                  
Accident Year 2015 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 433,888 435,438 434,254 425,865 419,663 390,002        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 413,838 400,304 372,587 327,462 267,420 160,237        
IBNR $ 3,111                  
Cumulative Number of Reported Claims | claim 53,000                  
Accident Year 2015 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 718,820 720,324 728,720 730,251 734,204 747,683        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 707,349 701,503 691,705 669,706 613,868 397,261        
IBNR $ 8,958                  
Cumulative Number of Reported Claims | claim 32,000                  
Accident Year 2015 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 306,296 296,297 255,400 233,443 234,862 262,448        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 206,864 179,884 142,334 91,949 48,833 17,952        
IBNR 35,570                  
Accident Year 2015 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 39,781 39,682 45,115 50,099 57,897 69,977        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,083 4,416 4,099 3,272 2,481 2,069        
IBNR 17,184                  
Accident Year 2015 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 131,669 130,005 130,888 132,399 118,026 127,852        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 125,939 123,023 118,977 109,378 89,366 $ 53,482        
IBNR 2,404                  
Accident Year 2016 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,046,823 1,032,780 1,021,524 1,012,851 1,019,851          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 678,209 559,218 390,664 209,217 69,579          
IBNR $ 171,325                  
Cumulative Number of Reported Claims | claim 27,000                  
Accident Year 2016 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 651,278 660,520 684,700 696,339 702,716          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 537,861 504,850 446,072 338,835 142,998          
IBNR $ 51,369                  
Cumulative Number of Reported Claims | claim 58,000                  
Accident Year 2016 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 442,521 405,390 364,312 327,706 313,365          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 299,813 257,221 202,906 103,321 28,833          
IBNR $ 49,740                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2016 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 443,338 446,521 445,516 434,117 434,582          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 412,967 393,509 344,503 282,375 186,867          
IBNR $ 8,135                  
Cumulative Number of Reported Claims | claim 52,000                  
Accident Year 2016 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 754,184 759,735 765,311 778,376 775,477          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 734,110 729,011 713,634 672,013 417,988          
IBNR $ 9,360                  
Cumulative Number of Reported Claims | claim 34,000                  
Accident Year 2016 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 304,845 270,833 248,367 256,011 243,970          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 172,994 141,301 100,862 62,094 19,998          
IBNR 45,117                  
Accident Year 2016 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 65,485 64,957 71,404 70,281 72,657          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 7,685 5,928 5,573 4,783 2,498          
IBNR 20,435                  
Accident Year 2016 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 186,786 181,429 182,171 174,935 168,634          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 176,509 169,013 157,834 133,805 $ 78,994          
IBNR 4,318                  
Accident Year 2017 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,140,639 1,123,930 1,101,376 1,067,556            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 639,977 453,973 256,345 80,163            
IBNR $ 253,192                  
Cumulative Number of Reported Claims | claim 27,000                  
Accident Year 2017 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 673,216 689,622 733,505 762,093            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 525,753 468,817 362,299 153,456            
IBNR $ 64,176                  
Cumulative Number of Reported Claims | claim 58,000                  
Accident Year 2017 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 379,408 340,569 334,682 335,565            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 244,396 163,731 97,034 36,839            
IBNR $ 85,159                  
Cumulative Number of Reported Claims | claim 10,000                  
Accident Year 2017 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 435,099 431,177 429,372 431,504            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 372,689 327,765 268,201 181,317            
IBNR $ 14,298                  
Cumulative Number of Reported Claims | claim 47,000                  
Accident Year 2017 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 747,739 748,399 754,217 754,135            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 731,440 719,286 690,343 445,835            
IBNR $ 12,056                  
Cumulative Number of Reported Claims | claim 42,000                  
Accident Year 2017 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 265,111 242,603 224,835 234,561            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 124,641 70,129 40,440 16,498            
IBNR 58,308                  
Accident Year 2017 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 71,025 70,749 80,508 76,701            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 17,327 15,356 12,810 6,282            
IBNR 27,947                  
Accident Year 2017 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 198,529 199,892 200,917 207,096            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 180,242 172,100 141,651 $ 72,180            
IBNR 8,040                  
Accident Year 2018 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,123,121 1,133,601 1,106,108              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 436,282 264,629 86,958              
IBNR $ 408,131                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2018 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 715,055 724,697 778,964              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 508,546 397,464 171,006              
IBNR $ 80,252                  
Cumulative Number of Reported Claims | claim 56,000                  
Accident Year 2018 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 335,852 324,934 337,865              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 156,102 100,284 28,432              
IBNR $ 125,091                  
Cumulative Number of Reported Claims | claim 10,000                  
Accident Year 2018 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 479,502 463,057 443,045              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 350,572 281,888 180,306              
IBNR $ 29,712                  
Cumulative Number of Reported Claims | claim 45,000                  
Accident Year 2018 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 747,396 750,123 761,177              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 709,202 662,657 415,529              
IBNR $ 23,631                  
Cumulative Number of Reported Claims | claim 48,000                  
Accident Year 2018 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 234,286 213,665 224,640              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 78,219 41,382 11,187              
IBNR 88,934                  
Accident Year 2018 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 71,448 72,505 77,820              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 9,368 8,230 6,141              
IBNR 34,366                  
Accident Year 2018 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 103,751 112,503 108,829              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 82,815 65,527 $ 34,195              
IBNR 2,423                  
Accident Year 2019 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,240,028 1,243,287                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 275,803 88,422                
IBNR $ 662,084                  
Cumulative Number of Reported Claims | claim 25,000                  
Accident Year 2019 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 721,018 784,281                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 397,376 184,715                
IBNR $ 141,657                  
Cumulative Number of Reported Claims | claim 53,000                  
Accident Year 2019 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 336,559 339,825                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 98,625 32,081                
IBNR $ 176,941                  
Cumulative Number of Reported Claims | claim 11,000                  
Accident Year 2019 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 488,777 483,483                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 290,449 185,488                
IBNR $ 74,684                  
Cumulative Number of Reported Claims | claim 44,000                  
Accident Year 2019 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 702,281 722,122                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 616,695 405,500                
IBNR $ 43,842                  
Cumulative Number of Reported Claims | claim 43,000                  
Accident Year 2019 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 234,813 240,408                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 39,435 14,641                
IBNR 142,399                  
Accident Year 2019 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 77,482 78,929                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 10,884 6,241                
IBNR 39,832                  
Accident Year 2019 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 77,618 103,749                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 54,864 $ 23,103                
IBNR 6,913                  
Accident Year 2020 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,342,065                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 72,274                  
IBNR $ 1,099,126                  
Cumulative Number of Reported Claims | claim 17,000                  
Accident Year 2020 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 725,245                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 172,478                  
IBNR $ 319,131                  
Cumulative Number of Reported Claims | claim 39,000                  
Accident Year 2020 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 398,429                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 28,474                  
IBNR $ 328,303                  
Cumulative Number of Reported Claims | claim 10,000                  
Accident Year 2020 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 523,753                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 142,826                  
IBNR $ 264,630                  
Cumulative Number of Reported Claims | claim 28,000                  
Accident Year 2020 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 901,698                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 460,656                  
IBNR $ 244,828                  
Cumulative Number of Reported Claims | claim 34,000                  
Accident Year 2020 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 305,114                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 20,845                  
IBNR 255,936                  
Accident Year 2020 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 84,354                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,869                  
IBNR 59,854                  
Accident Year 2020 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 115,116                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 26,637                  
IBNR $ 48,509