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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, 2023
USD ($)
claim
Dec. 31, 2022
USD ($)
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 ($)
Dec. 31, 2014
USD ($)
Claims Development [Line Items]                    
Reserves for loss and loss adjustment expenses, net of reinsurance $ 16,051,744                  
Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 13,419,768                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 7,202,094                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 154,001                  
Reserves for loss and loss adjustment expenses, net of reinsurance 6,371,675                  
Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 6,813,772                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,209,715                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 208,451                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,812,508                  
Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 4,149,302                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 2,216,817                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 38,814                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,971,299                  
Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 5,647,754                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,197,877                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 2,953                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,452,830                  
Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 8,042,131                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 7,276,034                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 5,171                  
Reserves for loss and loss adjustment expenses, net of reinsurance 771,268                  
Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 3,391,253                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,619,402                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 402,281                  
Reserves for loss and loss adjustment expenses, net of reinsurance 2,174,132                  
Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 622,374                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 127,434                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 576,491                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,071,431                  
Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,361,346                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,087,421                  
Reserves for loss and loss adjustment expenses before 2014, net of reinsurance 1,472                  
Reserves for loss and loss adjustment expenses, net of reinsurance 275,397                  
Accident Year 2014 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 863,342 $ 863,636 $ 862,440 $ 863,890 $ 868,639 $ 862,568 $ 849,657 $ 845,254 $ 847,308 $ 846,005
Cumulative paid claims and claim adjustment expenses, net of reinsurance 810,687 782,462 759,003 729,573 679,351 593,577 479,657 337,870 190,637 78,811
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 28,447                  
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 $ 589,283 598,977 602,000 600,194 604,030 615,435 617,242 627,767 637,307 639,436
Cumulative paid claims and claim adjustment expenses, net of reinsurance 553,739 547,894 538,914 531,475 521,141 503,915 471,235 412,611 319,743 148,405
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 12,555                  
Short-duration insurance contract, 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 $ 256,527 254,514 255,527 256,157 257,205 238,388 242,903 258,874 245,854 252,473
Cumulative paid claims and claim adjustment expenses, net of reinsurance 243,397 238,277 235,581 226,323 214,915 198,176 175,230 137,931 83,246 19,318
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 10,067                  
Short-duration insurance contract, cumulative number of reported claims | claim 7,000                  
Accident Year 2014 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 408,743 408,816 407,435 408,304 412,757 412,929 415,490 417,539 385,214 363,882
Cumulative paid claims and claim adjustment expenses, net of reinsurance 407,212 405,817 405,061 404,067 401,368 393,428 364,768 327,548 237,622 155,553
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 291                  
Short-duration insurance contract, 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 $ 656,326 656,931 658,600 660,388 657,968 657,637 656,704 658,437 701,133 697,102
Cumulative paid claims and claim adjustment expenses, net of reinsurance 652,449 652,379 652,054 651,560 649,068 641,861 626,751 608,396 590,511 369,514
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,483                  
Short-duration insurance contract, cumulative number of reported claims | claim 30,000                  
Accident Year 2014 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 343,235 341,244 336,622 335,560 323,855 324,470 330,354 318,559 319,210 319,818
Cumulative paid claims and claim adjustment expenses, net of reinsurance 296,469 283,876 271,062 251,736 227,038 197,644 154,811 115,585 68,906 21,302
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 13,680                  
Accident Year 2014 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 32,185 35,707 40,652 42,618 42,541 41,671 45,758 49,478 57,558 63,465
Cumulative paid claims and claim adjustment expenses, net of reinsurance 14,632 13,491 11,938 11,154 7,595 5,808 4,175 3,354 1,729 358
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 3,400                  
Accident Year 2014 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 96,135 96,648 97,308 99,244 98,846 99,200 99,973 97,261 96,586 113,055
Cumulative paid claims and claim adjustment expenses, net of reinsurance 94,996 95,620 95,186 94,552 93,087 91,376 88,240 82,113 66,816 $ 38,734
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 344                  
Accident Year 2015 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,018,418 1,011,753 982,145 975,534 965,497 963,237 960,136 985,529 949,853  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 928,793 873,489 814,964 756,353 675,274 537,415 381,682 210,421 82,664  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 42,923                  
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 $ 603,732 612,687 617,478 620,741 626,432 641,169 650,997 690,525 712,800  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 564,658 557,215 544,849 531,512 512,933 477,541 421,734 323,744 139,320  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 18,013                  
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 $ 286,320 286,950 282,806 282,452 291,157 275,604 274,233 257,634 259,122  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 258,988 246,501 239,514 232,506 215,821 187,007 139,396 85,169 20,319  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 13,275                  
Short-duration insurance contract, cumulative number of reported claims | claim 8,000                  
Accident Year 2015 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 428,339 428,898 427,079 429,950 431,514 430,503 422,344 416,251 389,529  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 421,749 419,891 416,060 410,275 396,832 369,223 324,175 264,377 159,932  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,013                  
Short-duration insurance contract, 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 $ 714,974 710,266 709,937 711,665 713,225 721,082 722,556 725,927 737,494  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 716,682 707,447 707,271 700,569 694,780 685,314 663,402 608,056 392,762  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 2,044                  
Short-duration insurance contract, cumulative number of reported claims | claim 32,000                  
Accident Year 2015 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 313,327 308,986 304,503 303,053 293,032 252,249 230,136 231,574 259,244  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 266,497 250,504 233,281 204,981 178,280 141,073 91,016 48,371 17,876  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 16,235                  
Accident Year 2015 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 27,590 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 6,844 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 4,664                  
Accident Year 2015 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 131,947 131,393 130,697 131,133 129,472 130,389 131,722 117,503 127,315  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 128,947 128,037 126,895 125,494 122,630 118,615 109,053 89,120 $ 53,435  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,007                  
Accident Year 2016 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,134,162 1,092,036 1,060,478 1,044,563 1,030,490 1,019,067 1,010,016 1,017,074    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 955,951 871,252 766,774 676,957 558,176 389,988 208,851 69,404    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 80,989                  
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                  
Accident Year 2016 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 641,549 654,385 657,972 651,278 660,520 684,700 696,339 702,716    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 584,330 572,669 558,934 537,861 504,850 446,072 338,835 142,998    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 19,823                  
Short-duration insurance contract, 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 $ 449,678 463,625 468,154 438,761 401,569 360,778 324,043 310,140    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 412,645 405,052 357,094 297,088 254,994 201,167 102,232 28,503    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 15,850                  
Short-duration insurance contract, cumulative number of reported claims | claim 9,000                  
Accident Year 2016 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 442,164 440,412 438,616 438,917 442,122 441,051 429,762 430,132    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 433,882 427,662 419,203 408,676 389,233 340,319 278,289 183,777    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 2,410                  
Short-duration insurance contract, 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 $ 752,960 752,541 753,926 751,510 757,010 762,449 775,358 771,829    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 744,157 739,143 738,302 731,552 726,462 711,238 669,738 416,513    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 3,288                  
Short-duration insurance contract, cumulative number of reported claims | claim 34,000                  
Accident Year 2016 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 309,126 310,273 301,242 301,629 267,808 245,317 252,786 240,868    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 241,884 224,739 205,052 171,465 140,104 100,074 61,710 19,884    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 19,483                  
Accident Year 2016 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 53,304 61,432 65,222 65,485 64,957 71,404 70,281 72,657    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 13,569 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 6,192                  
Accident Year 2016 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 186,176 185,133 184,053 186,086 180,816 181,595 174,440 167,998    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 184,208 182,300 177,961 175,867 168,433 157,341 133,478 $ 78,887    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,303                  
Accident Year 2017 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,267,359 1,249,086 1,178,596 1,138,821 1,121,979 1,099,566 1,065,837      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,048,203 930,529 774,674 638,879 453,073 255,591 79,937      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 95,118                  
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 $ 675,871 682,153 683,880 673,216 689,622 733,505 762,093      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 603,006 583,258 559,198 525,753 468,817 362,299 153,456      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 22,933                  
Short-duration insurance contract, 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 $ 401,661 393,054 384,107 377,209 338,526 332,317 333,255      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 329,303 306,535 261,033 242,929 162,660 96,179 36,450      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 45,095                  
Short-duration insurance contract, cumulative number of reported claims | claim 10,000                  
Accident Year 2017 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 445,263 444,483 440,004 434,186 430,205 428,420 430,445      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 431,165 419,548 401,846 371,755 326,851 267,309 180,494      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 3,148                  
Short-duration insurance contract, 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 $ 746,406 747,710 746,475 746,878 747,521 753,274 752,865      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 746,917 741,582 734,415 730,584 718,436 689,587 445,208      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 4,267                  
Short-duration insurance contract, cumulative number of reported claims | claim 42,000                  
Accident Year 2017 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 314,895 298,284 281,132 261,438 238,916 220,878 231,313      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 207,378 174,745 147,049 123,404 69,190 40,013 16,457      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 28,441                  
Accident Year 2017 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 55,713 62,647 66,795 71,025 70,749 80,508 76,701      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 21,275 19,275 18,375 17,327 15,356 12,810 6,282      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 9,442                  
Accident Year 2017 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 193,551 192,458 191,941 198,039 199,481 200,487 206,666      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 188,497 185,895 182,680 179,799 171,736 141,390 $ 72,114      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,452                  
Accident Year 2018 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,299,691 1,232,803 1,156,233 1,121,429 1,131,672 1,104,011        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 986,108 806,757 615,688 435,711 264,261 86,788        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 141,108                  
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                  
Accident Year 2018 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 715,018 721,170 724,056 715,055 724,697 778,964        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 642,292 613,675 574,889 508,546 397,464 171,006        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 27,179                  
Short-duration insurance contract, 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 $ 397,593 382,239 359,493 333,413 322,297 335,128        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 283,639 244,068 198,546 155,102 99,540 28,108        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 81,991                  
Short-duration insurance contract, cumulative number of reported claims | claim 10,000                  
Accident Year 2018 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 530,374 521,794 494,442 479,085 462,657 442,701        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 498,094 463,232 412,985 350,212 281,564 180,110        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 6,234                  
Short-duration insurance contract, 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 $ 741,264 742,212 744,467 746,183 748,859 759,748        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 730,199 725,128 725,027 708,101 661,607 414,833        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 7,835                  
Short-duration insurance contract, cumulative number of reported claims | claim 48,000                  
Accident Year 2018 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 292,927 261,004 246,861 230,817 210,492 221,393        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 182,326 141,309 109,290 77,371 40,884 11,056        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 37,287                  
Accident Year 2018 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 50,744 57,847 66,180 71,448 72,505 77,820        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 13,583 12,414 10,359 9,368 8,230 6,141        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 12,964                  
Accident Year 2018 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 101,498 103,015 105,165 103,192 112,133 108,348        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 96,262 94,931 87,789 82,327 65,071 $ 33,972        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,898                  
Accident Year 2019 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,371,217 1,294,768 1,237,950 1,237,516 1,240,667          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 909,617 704,851 471,186 275,315 88,173          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 205,380                  
Short-duration insurance contract, cumulative number of reported claims | claim 29,000                  
Accident Year 2019 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 722,456 734,034 732,762 721,018 784,281          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 618,324 581,003 515,914 397,376 184,715          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 39,081                  
Short-duration insurance contract, cumulative number of reported claims | claim 54,000                  
Accident Year 2019 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 362,983 354,121 345,548 332,426 336,397          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 235,124 200,259 147,767 97,302 31,635          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 89,944                  
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                  
Accident Year 2019 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 548,688 530,967 504,901 488,379 483,120          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 495,111 440,449 374,545 290,194 185,289          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 11,878                  
Short-duration insurance contract, cumulative number of reported claims | claim 45,000                  
Accident Year 2019 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 684,707 684,606 690,928 701,168 721,072          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 665,789 657,699 645,251 615,748 404,870          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 14,592                  
Short-duration insurance contract, cumulative number of reported claims | claim 43,000                  
Accident Year 2019 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 272,393 240,879 239,206 230,620 236,517          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 142,501 94,735 63,945 39,035 14,544          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 52,998                  
Accident Year 2019 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 66,071 73,978 76,242 77,482 78,929          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 18,836 15,436 12,728 10,884 6,241          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 12,977                  
Accident Year 2019 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 81,157 81,192 82,036 77,255 103,316          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 73,234 71,264 68,693 54,709 $ 23,129          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 2,479                  
Accident Year 2020 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,167,753 1,158,966 1,212,875 1,339,702            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 621,085 423,186 224,991 72,203            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 317,212                  
Short-duration insurance contract, cumulative number of reported claims | claim 23,000                  
Accident Year 2020 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 668,222 704,008 716,430 725,245            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 548,585 485,203 380,454 172,478            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 36,045                  
Short-duration insurance contract, cumulative number of reported claims | claim 42,000                  
Accident Year 2020 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 313,386 337,796 375,559 394,304            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 169,000 128,869 80,180 27,996            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 100,150                  
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                  
Accident Year 2020 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 469,184 442,165 428,761 523,739            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 374,990 308,452 228,358 142,816            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 16,227                  
Short-duration insurance contract, cumulative number of reported claims | claim 30,000                  
Accident Year 2020 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 926,379 922,258 904,545 900,816            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 900,682 845,590 784,553 460,351            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 7,624                  
Short-duration insurance contract, cumulative number of reported claims | claim 38,000                  
Accident Year 2020 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 313,904 290,037 293,574 299,888            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 137,043 81,710 49,637 20,746            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 103,742                  
Accident Year 2020 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 72,376 80,452 83,468 84,354            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 12,869 10,471 8,699 4,869            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 26,582                  
Accident Year 2020 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 121,417 116,786 117,885 114,645            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 95,643 86,885 65,563 $ 26,606            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 4,291                  
Accident Year 2021 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,347,336 1,390,800 1,534,634              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 492,271 267,564 76,568              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 537,640                  
Short-duration insurance contract, cumulative number of reported claims | claim 25,000                  
Accident Year 2021 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 667,517 701,703 742,687              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 490,648 384,867 172,729              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 67,127                  
Short-duration insurance contract, cumulative number of reported claims | claim 46,000                  
Accident Year 2021 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 446,596 471,456 525,389              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 153,063 85,886 28,542              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 215,820                  
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                  
Accident Year 2021 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 632,918 596,810 614,422              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 429,419 319,941 180,860              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 54,629                  
Short-duration insurance contract, cumulative number of reported claims | claim 38,000                  
Accident Year 2021 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 815,228 832,240 828,347              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 754,448 698,030 405,461              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 24,478                  
Short-duration insurance contract, cumulative number of reported claims | claim 36,000                  
Accident Year 2021 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 344,272 347,054 360,399              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 89,076 43,791 10,910              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 190,749                  
Accident Year 2021 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 77,220 87,980 98,110              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 8,872 6,026 4,586              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 31,581                  
Accident Year 2021 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 144,429 146,726 133,989              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 109,179 71,800 $ 15,245              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 13,709                  
Accident Year 2022 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,830,125 1,823,703                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 356,621 93,477                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,116,619                  
Short-duration insurance contract, cumulative number of reported claims | claim 25,000                  
Accident Year 2022 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 745,218 772,620                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 408,929 180,982                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 153,476                  
Short-duration insurance contract, cumulative number of reported claims | claim 46,000                  
Accident Year 2022 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 586,327 649,725                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 90,594 33,417                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 405,848                  
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                  
Accident Year 2022 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 825,359 792,553                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 428,621 253,206                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 192,025                  
Short-duration insurance contract, cumulative number of reported claims | claim 44,000                  
Accident Year 2022 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 928,562 945,041                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 799,125 472,020                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 54,776                  
Short-duration insurance contract, cumulative number of reported claims | claim 36,000                  
Accident Year 2022 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 437,358 447,231                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 45,625 11,574                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 325,597                  
Accident Year 2022 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 76,725 98,923                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 10,564 5,898                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 35,307                  
Accident Year 2022 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 167,351 167,223                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 91,718 $ 25,515                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 30,362                  
Accident Year 2023 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 2,120,365                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 92,758                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,815,051                  
Short-duration insurance contract, cumulative number of reported claims | claim 19,000                  
Accident Year 2023 | Workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 784,906                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 195,204                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 352,511                  
Short-duration insurance contract, cumulative number of reported claims | claim 43,000                  
Accident Year 2023 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 648,231                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 41,064                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 509,380                  
Short-duration insurance contract, cumulative number of reported claims | claim 12,000                  
Accident Year 2023 | Auto                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 916,722                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 277,634                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 444,715                  
Short-duration insurance contract, cumulative number of reported claims | claim 40,000                  
Accident Year 2023 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 1,075,325                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 565,586                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 286,870                  
Short-duration insurance contract, cumulative number of reported claims | claim 27,000                  
Accident Year 2023 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 449,816                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 10,603                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 396,340                  
Accident Year 2023 | Monoline excess                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 110,446                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 6,390                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 73,881                  
Accident Year 2023 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 137,685                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 24,737                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 60,458