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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, 2022
USD ($)
claim
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Claims Development [Line Items]                    
Reserves for loss and loss adjustment expenses, net of reinsurance $ 14,665,237                  
Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 11,924,316                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,513,546                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 125,357                  
Reserves for loss and loss adjustment expenses, net of reinsurance 5,536,127                  
Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 6,716,685                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,008,947                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 214,233                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,921,971                  
Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 3,874,102                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 2,161,200                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 36,484                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,749,386                  
Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 4,968,723                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 3,819,737                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 3,731                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,152,717                  
Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 7,538,091                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,781,190                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 4,727                  
Reserves for loss and loss adjustment expenses, net of reinsurance 761,628                  
Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 3,147,324                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,572,474                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 417,861                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,992,711                  
Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 612,376                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 110,777                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 644,712                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,146,311                  
Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,325,878                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,047,114                  
Reserves for loss and loss adjustment expenses before 2013, net of reinsurance 1,036                  
Reserves for loss and loss adjustment expenses, net of reinsurance 279,800                  
Accident Year 2011 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 809,358 $ 807,998 $ 805,915 $ 801,689 $ 806,914 $ 800,593 $ 779,706 $ 779,908 $ 788,581 $ 749,261
Cumulative paid claims and claim adjustment expenses, net of reinsurance 761,461 738,858 717,908 691,495 646,257 585,608 470,566 330,127 188,023 63,243
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 21,362                  
Short-duration insurance contract, cumulative number of reported claims | claim 26,000                  
Accident Year 2011 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 534,948 538,503 540,322 541,926 542,274 547,000 543,238 546,995 547,295 552,570
Cumulative paid claims and claim adjustment expenses, net of reinsurance 502,181 496,809 489,075 479,104 466,580 447,894 414,160 363,028 277,538 117,900
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 11,473                  
Short-duration insurance contract, cumulative number of reported claims | claim 53,000                  
Accident Year 2011 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 283,216 279,870 279,824 281,888 276,566 267,978 246,943 241,046 244,556 265,061
Cumulative paid claims and claim adjustment expenses, net of reinsurance 273,094 267,776 261,106 255,863 246,616 204,955 176,079 118,767 63,503 23,295
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 4,681                  
Short-duration insurance contract, cumulative number of reported claims | claim 7,000                  
Accident Year 2011 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 364,607 364,439 364,207 365,275 366,166 366,646 376,243 368,894 349,136 327,514
Cumulative paid claims and claim adjustment expenses, net of reinsurance 364,147 362,624 362,252 361,499 352,792 342,961 321,855 267,253 218,596 142,929
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 103                  
Short-duration insurance contract, cumulative number of reported claims | claim 44,000                  
Accident Year 2011 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 541,728 541,857 542,148 542,863 544,681 548,169 549,518 565,810 573,905 565,391
Cumulative paid claims and claim adjustment expenses, net of reinsurance 539,081 538,985 537,582 536,416 535,756 534,569 527,789 526,031 480,140 310,750
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,125                  
Short-duration insurance contract, cumulative number of reported claims | claim 25,000                  
Accident Year 2011 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 302,135 302,672 301,415 302,595 298,317 291,105 282,307 272,475 269,340 318,365
Cumulative paid claims and claim adjustment expenses, net of reinsurance 263,467 254,273 241,573 225,882 205,553 178,060 144,331 108,852 63,745 28,945
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 12,543                  
Accident Year 2011 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 23,306 25,701 29,758 31,752 35,120 40,207 44,162 48,143 50,355 63,995
Cumulative paid claims and claim adjustment expenses, net of reinsurance 9,490 8,805 6,476 5,349 4,418 3,396 3,008 3,588 1,897 647
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 5,184                  
Accident Year 2011 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 105,711 106,316 107,552 109,697 111,890 112,572 111,937 114,100 112,616 141,380
Cumulative paid claims and claim adjustment expenses, net of reinsurance 105,059 104,388 104,403 107,603 106,043 104,323 101,538 92,625 74,572 $ 36,577
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 72                  
Accident Year 2012 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 862,630 861,382 862,749 867,400 861,313 848,138 843,501 845,564 844,832  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 781,533 758,098 728,718 678,490 592,828 478,924 337,426 190,419 78,705  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 35,808                  
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                  
Accident Year 2012 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 598,977 602,000 600,194 604,030 615,435 617,242 627,767 637,307 639,436  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 547,894 538,914 531,475 521,141 503,915 471,235 412,611 319,743 148,405  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 21,528                  
Short-duration insurance contract, cumulative number of reported claims | claim 57,000                  
Accident Year 2012 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 253,244 254,243 254,868 255,850 236,961 241,376 257,309 244,574 250,740  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 237,145 234,459 225,236 213,888 197,272 174,524 137,341 83,063 19,225  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 12,284                  
Short-duration insurance contract, 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 $ 407,970 406,589 407,470 411,920 412,120 414,732 416,802 385,241 363,891  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 405,004 404,259 403,273 400,577 392,658 364,054 326,854 237,648 155,564  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 403                  
Short-duration insurance contract, cumulative number of reported claims | claim 47,000                  
Accident Year 2012 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 658,557 660,250 662,012 659,568 659,230 658,252 659,563 697,845 691,971  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 654,089 653,784 653,272 650,816 643,619 628,440 609,948 587,607 368,865  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,549                  
Short-duration insurance contract, cumulative number of reported claims | claim 30,000                  
Accident Year 2012 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 341,649 336,990 335,883 324,150 324,693 330,382 318,443 318,904 319,454  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 284,360 271,512 252,155 227,448 197,996 155,070 115,779 68,374 21,297  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 16,184                  
Accident Year 2012 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 35,707 40,652 42,618 42,541 41,671 45,823 49,570 57,650 63,561  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 13,491 11,938 11,154 7,595 5,808 4,175 3,354 2,341 377  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 7,484                  
Accident Year 2012 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 96,656 97,315 99,244 98,838 99,176 99,941 97,195 96,492 112,907  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 95,633 95,198 94,565 93,099 91,390 88,257 82,119 66,829 $ 38,780  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 177                  
Accident Year 2013 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,010,796 981,071 974,345 964,301 961,761 958,734 984,262 948,389    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 872,657 814,191 755,641 674,665 536,855 381,328 210,151 82,638    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 55,430                  
Short-duration insurance contract, cumulative number of reported claims | claim 27,000                  
Accident Year 2013 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 612,687 617,478 620,741 626,432 641,169 650,997 690,525 712,800    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 557,215 544,849 531,512 512,933 477,541 421,734 323,744 139,320    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 29,213                  
Short-duration insurance contract, cumulative number of reported claims | claim 58,000                  
Accident Year 2013 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 286,271 282,065 281,718 290,141 274,546 272,899 256,595 257,946    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 246,031 239,150 232,143 215,408 186,688 139,205 85,047 20,331    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 19,949                  
Short-duration insurance contract, cumulative number of reported claims | claim 8,000                  
Accident Year 2013 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 427,923 426,107 428,981 430,557 429,608 421,522 415,446 389,577    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 418,993 415,170 409,426 396,018 368,448 323,429 263,663 159,987    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,362                  
Short-duration insurance contract, cumulative number of reported claims | claim 53,000                  
Accident Year 2013 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 710,978 710,663 712,361 713,899 721,789 723,206 726,287 736,523    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 708,214 708,065 701,341 695,563 686,029 664,084 608,748 392,921    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 3,842                  
Short-duration insurance contract, cumulative number of reported claims | claim 32,000                  
Accident Year 2013 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 309,317 304,805 303,261 293,094 252,277 230,085 231,430 259,019    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 250,880 233,643 205,289 178,521 141,254 91,140 48,442 17,888    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 19,728                  
Accident Year 2013 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 30,104 36,774 39,781 39,682 45,115 50,099 57,897 69,977    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,457 5,421 5,083 4,416 4,099 3,272 2,481 2,069    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 8,683                  
Accident Year 2013 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 131,342 130,642 131,071 129,398 130,301 131,625 117,452 127,118    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 127,995 126,848 125,439 122,566 118,552 109,051 89,121 $ 53,474    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,061                  
Accident Year 2014 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,091,569 1,059,897 1,043,948 1,029,821 1,018,253 1,009,051 1,016,106      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 870,981 766,515 676,735 557,998 389,862 208,828 69,407      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 96,310                  
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                  
Accident Year 2014 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 654,385 657,972 651,278 660,520 684,700 696,339 702,716      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 572,669 558,934 537,861 504,850 446,072 338,835 142,998      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 36,834                  
Short-duration insurance contract, cumulative number of reported claims | claim 58,000                  
Accident Year 2014 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 463,102 467,545 438,065 400,799 360,110 323,222 309,417      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 404,742 356,812 296,863 254,872 201,019 102,173 28,517      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 33,241                  
Short-duration insurance contract, cumulative number of reported claims | claim 9,000                  
Accident Year 2014 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 439,682 437,884 438,192 441,408 440,334 429,074 429,329      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 426,955 418,499 407,989 388,554 339,657 277,665 183,160      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 2,244                  
Short-duration insurance contract, cumulative number of reported claims | claim 52,000                  
Accident Year 2014 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 752,604 753,952 751,530 757,009 762,460 775,308 771,390      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 739,244 738,390 731,651 726,549 711,385 669,891 416,611      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 3,363                  
Short-duration insurance contract, cumulative number of reported claims | claim 34,000                  
Accident Year 2014 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 310,451 301,355 301,663 267,850 245,268 252,638 240,655      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 225,077 205,354 171,719 140,299 100,200 61,770 19,904      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 24,928                  
Accident Year 2014 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 61,432 65,222 65,485 64,957 71,404 70,281 72,657      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 11,819 9,883 7,685 5,928 5,573 4,783 2,498      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 13,634                  
Accident Year 2014 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 185,249 184,150 186,159 180,885 181,634 174,423 167,901      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 182,411 178,060 175,949 168,506 157,404 133,516 $ 78,920      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,482                  
Accident Year 2015 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,249,129 1,178,604 1,138,785 1,121,833 1,099,396 1,065,550        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 930,630 774,738 638,934 453,097 255,603 79,887        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 142,974                  
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                  
Accident Year 2015 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 682,153 683,880 673,216 689,622 733,505 762,093        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 583,258 559,198 525,753 468,817 362,299 153,456        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 41,944                  
Short-duration insurance contract, 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 $ 393,409 384,416 377,410 338,723 332,400 333,267        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 306,713 261,225 243,088 162,829 96,312 36,503        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 53,786                  
Short-duration insurance contract, cumulative number of reported claims | claim 10,000                  
Accident Year 2015 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 444,472 439,991 434,170 430,198 428,419 430,440        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 419,545 401,844 371,761 326,861 267,326 180,545        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 4,800                  
Short-duration insurance contract, cumulative number of reported claims | claim 47,000                  
Accident Year 2015 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 747,869 746,603 747,003 747,630 753,326 752,727        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 741,685 734,509 730,688 718,538 689,662 445,285        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 8,195                  
Short-duration insurance contract, cumulative number of reported claims | claim 42,000                  
Accident Year 2015 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 298,486 281,254 261,482 238,883 220,699 231,082        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 175,059 147,311 123,614 69,350 40,085 16,469        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 36,491                  
Accident Year 2015 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 62,647 66,795 71,025 70,749 80,508 76,701        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 19,275 18,375 17,327 15,356 12,810 6,282        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 16,529                  
Accident Year 2015 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 192,379 191,867 197,978 199,410 200,394 206,560        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 185,823 182,609 179,749 171,675 141,340 $ 72,126        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,209                  
Accident Year 2016 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,232,803 1,156,157 1,121,317 1,131,549 1,103,900          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 806,869 615,753 435,729 264,299 86,798          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 181,880                  
Short-duration insurance contract, 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 $ 721,170 724,056 715,055 724,697 778,964          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 613,675 574,889 508,546 397,464 171,006          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 43,766                  
Short-duration insurance contract, cumulative number of reported claims | claim 56,000                  
Accident Year 2016 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 382,409 359,566 333,408 322,176 334,848          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 244,284 198,697 155,212 99,598 28,101          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 73,828                  
Short-duration insurance contract, 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 $ 521,667 494,315 478,966 462,544 442,610          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 463,117 412,874 350,110 281,475 180,056          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 10,720                  
Short-duration insurance contract, cumulative number of reported claims | claim 46,000                  
Accident Year 2016 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 742,289 744,544 746,265 748,931 759,634          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 725,228 725,138 708,214 661,741 414,910          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 11,491                  
Short-duration insurance contract, cumulative number of reported claims | claim 48,000                  
Accident Year 2016 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 261,148 246,898 230,790 210,397 221,193          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 141,553 109,474 77,498 40,953 11,076          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 44,474                  
Accident Year 2016 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 57,847 66,180 71,448 72,505 77,820          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 12,414 10,359 9,368 8,230 6,141          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 20,826                  
Accident Year 2016 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 102,953 105,101 103,104 112,068 108,220          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 94,870 87,736 82,259 65,079 $ 33,991          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,754                  
Accident Year 2017 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,294,681 1,237,824 1,237,336 1,240,560            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 704,928 471,239 275,343 88,195            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 292,625                  
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                  
Accident Year 2017 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 734,034 732,762 721,018 784,281            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 581,003 515,914 397,376 184,715            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 67,254                  
Short-duration insurance contract, cumulative number of reported claims | claim 54,000                  
Accident Year 2017 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 354,283 345,614 332,385 336,129            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 200,521 147,985 97,466 31,674            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 93,387                  
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                  
Accident Year 2017 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 530,876 504,813 488,291 483,019            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 440,378 374,479 290,124 185,236            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 22,483                  
Short-duration insurance contract, cumulative number of reported claims | claim 45,000                  
Accident Year 2017 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 684,708 691,015 701,241 721,073            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 657,819 645,374 615,869 404,975            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 18,311                  
Short-duration insurance contract, cumulative number of reported claims | claim 43,000                  
Accident Year 2017 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 240,848 239,131 230,460 236,318            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 94,856 64,020 39,091 14,560            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 72,504                  
Accident Year 2017 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 73,978 76,242 77,482 78,929            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 15,436 12,728 10,884 6,241            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 21,878                  
Accident Year 2017 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 81,014 81,858 77,062 103,113            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 71,054 68,457 54,499 $ 23,081            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 3,637                  
Accident Year 2018 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,158,880 1,212,790 1,339,565              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 423,283 225,068 72,232              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 489,718                  
Short-duration insurance contract, cumulative number of reported claims | claim 22,000                  
Accident Year 2018 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 704,008 716,430 725,245              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 485,203 380,454 172,478              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 96,322                  
Short-duration insurance contract, cumulative number of reported claims | claim 42,000                  
Accident Year 2018 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 337,961 375,577 394,107              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 129,168 80,408 28,106              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 159,767                  
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                  
Accident Year 2018 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 442,163 428,759 523,736              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 308,451 228,357 142,815              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 31,172                  
Short-duration insurance contract, cumulative number of reported claims | claim 30,000                  
Accident Year 2018 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 922,333 904,580 900,683              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 845,714 784,670 460,434              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 24,934                  
Short-duration insurance contract, cumulative number of reported claims | claim 38,000                  
Accident Year 2018 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 289,878 293,345 299,602              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 81,789 49,664 20,750              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 136,865                  
Accident Year 2018 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 80,452 83,468 84,354              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 10,471 8,699 4,869              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 35,638                  
Accident Year 2018 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 116,774 117,867 114,590              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 86,888 65,575 $ 26,574              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 3,946                  
Accident Year 2019 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,390,790 1,534,580                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 267,685 76,612                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 841,854                  
Short-duration insurance contract, cumulative number of reported claims | claim 23,000                  
Accident Year 2019 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 701,703 742,687                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 384,867 172,729                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 149,447                  
Short-duration insurance contract, cumulative number of reported claims | claim 45,000                  
Accident Year 2019 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 471,266 524,879                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 86,056 28,586                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 315,877                  
Short-duration insurance contract, 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 $ 596,810 614,422                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 319,941 180,860                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 111,412                  
Short-duration insurance contract, cumulative number of reported claims | claim 38,000                  
Accident Year 2019 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 832,183 828,187                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 698,092 405,512                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 57,246                  
Short-duration insurance contract, cumulative number of reported claims | claim 36,000                  
Accident Year 2019 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 346,736 359,952                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 43,838 10,918                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 243,222                  
Accident Year 2019 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 87,980 98,110                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,026 4,586                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 48,670                  
Accident Year 2019 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 146,761 133,938                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 71,797 $ 15,235                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 28,634                  
Accident Year 2020 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,823,680                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 93,519                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,507,284                  
Short-duration insurance contract, cumulative number of reported claims | claim 20,000                  
Accident Year 2020 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 772,620                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 180,982                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 348,348                  
Short-duration insurance contract, cumulative number of reported claims | claim 41,000                  
Accident Year 2020 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 648,941                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 33,446                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 543,483                  
Short-duration insurance contract, 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 $ 792,553                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 253,206                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 342,905                  
Short-duration insurance contract, cumulative number of reported claims | claim 40,000                  
Accident Year 2020 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 944,842                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 472,024                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 235,121                  
Short-duration insurance contract, cumulative number of reported claims | claim 32,000                  
Accident Year 2020 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 446,676                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 11,595                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 402,204                  
Accident Year 2020 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 98,923                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,898                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 66,865                  
Accident Year 2020 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 167,039                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 25,584                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 78,818