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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, 2024
USD ($)
claim
Dec. 31, 2023
USD ($)
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 $ 17,571,201                    
Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 14,994,380                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 8,033,653                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 178,312                    
Reserves for loss and loss adjustment expenses, net of reinsurance 7,139,039                    
Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 6,924,123                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,352,614                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 246,975                    
Reserves for loss and loss adjustment expenses, net of reinsurance 1,818,484                    
Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 4,440,919                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 2,362,092                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 49,908                    
Reserves for loss and loss adjustment expenses, net of reinsurance 2,128,735                    
Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 6,068,948                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,414,794                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 2,295                    
Reserves for loss and loss adjustment expenses, net of reinsurance 1,656,449                    
Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 8,365,242                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 7,401,278                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 5,937                    
Reserves for loss and loss adjustment expenses, net of reinsurance 969,901                    
Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 3,804,431                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,994,684                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 357,939                    
Reserves for loss and loss adjustment expenses, net of reinsurance 2,167,686                    
Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 728,836                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 146,654                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 577,639                    
Reserves for loss and loss adjustment expenses, net of reinsurance 1,159,821                    
Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,707,893                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,359,406                    
Reserves for loss and loss adjustment expenses before 2015, net of reinsurance 1,971                    
Reserves for loss and loss adjustment expenses, net of reinsurance 350,458                    
Accident Year 2014 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,019,173 $ 1,005,801 $ 999,316 $ 969,789 $ 963,118 $ 951,981 $ 948,706 $ 944,351 $ 969,246 $ 934,138  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 948,434 916,787 862,557 804,340 745,994 665,272 528,935 374,666 205,207 80,079  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 32,718                    
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 $ 604,806 603,731 612,687 617,477 620,741 626,432 641,169 650,997 690,525 712,800  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 570,125 564,658 557,215 544,849 531,512 512,933 477,541 421,734 323,744 139,320  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 14,681                    
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 $ 285,252 285,591 286,203 281,906 281,461 289,722 274,229 272,360 255,638 256,877  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 265,643 258,640 246,160 239,162 232,187 215,640 186,834 139,244 85,061 20,271  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 11,809                    
Short-duration insurance contract, cumulative number of reported claims | claim 8,000                    
Accident Year 2014 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 410,218 409,329 409,797 407,874 410,762 412,416 411,886 403,449 398,090 374,801  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 405,009 402,938 401,130 397,747 392,093 379,779 353,772   310,923 255,990 $ 155,699
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 784                    
Short-duration insurance contract, cumulative number of reported claims | claim 50,000                    
Accident Year 2014 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 697,949 699,018 694,284 693,963 695,584 697,123 704,501 705,938 708,711 718,877  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 702,432 700,792 691,545 691,365 684,679 678,891 669,556 647,710 592,871 380,892  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,485                    
Short-duration insurance contract, cumulative number of reported claims | claim 29,000                    
Accident Year 2014 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 346,178 336,653 335,541 333,368 331,370 321,934 281,470 260,696 261,479 285,491  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 308,837 295,608 278,540 260,577 232,001 203,940 163,728 110,230 61,106 24,672  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 9,461                    
Accident Year 2014 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 28,220 33,590 32,604 36,774 39,781 39,682 45,115 50,099 57,897 69,977  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 7,402 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 5,749                    
Accident Year 2014 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 146,485 146,893 146,346 145,697 146,140 144,486 145,422 146,437 132,570 141,484  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 144,369 143,999 143,103 141,963 140,564 137,708 133,605 123,995 103,634 64,495  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 444                    
Accident Year 2015 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,122,233 1,121,260 1,079,436 1,048,121 1,032,281 1,018,025 1,005,560 995,148 1,003,192    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 996,649 944,014 860,423 757,110 667,761 550,589 383,909 203,890 67,290    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 59,052                    
Short-duration insurance contract, cumulative number of reported claims | claim 29,000                    
Accident Year 2015 | Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 639,412 641,549 654,385 657,972 651,278 660,520 684,700 696,339 702,716    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 591,005 584,330 572,669 558,934 537,861 504,850 446,072 338,835 142,998    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 15,340                    
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 $ 452,769 449,033 462,868 467,183 437,690 400,178 359,228 322,107 307,902    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 421,639 412,243 404,669 356,717 296,742 254,694 200,949 102,079 28,432    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 15,280                    
Short-duration insurance contract, cumulative number of reported claims | claim 10,000                    
Accident Year 2015 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 425,280 426,814 425,063 423,171 423,216 426,365 425,808 415,208 414,205    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 421,309 418,672 412,624 404,241 394,720 375,891 330,222   270,304 $ 178,350  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,648                    
Short-duration insurance contract, cumulative number of reported claims | claim 48,000                    
Accident Year 2015 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 732,544 734,306 733,993 735,299 732,853 738,577 744,208 756,684 752,238    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 727,943 725,721 720,716 719,883 713,193 708,340 693,535 652,336 403,529    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,814                    
Short-duration insurance contract, cumulative number of reported claims | claim 31,000                    
Accident Year 2015 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 332,140 328,079 332,538 325,590 325,777 292,188 269,643 277,255 265,059    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 279,490 267,375 249,017 228,163 193,167 159,651 115,017 73,546 26,599    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 12,449                    
Accident Year 2015 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 55,371 59,804 63,932 65,222 65,485 64,957 71,404 70,281 72,657    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 16,872 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 5,800                    
Accident Year 2015 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 203,198 204,190 203,070 201,996 204,004 198,523 198,909 191,947 185,533    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 202,409 201,985 200,070 195,736 193,595 185,937 174,449 150,326 91,514    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 781                    
Accident Year 2016 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,289,349 1,256,832 1,238,759 1,169,138 1,129,392 1,111,550 1,088,787 1,055,253      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,119,123 1,038,695 922,090 767,665 632,721 447,743 251,172 77,719      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 79,218                    
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 $ 669,988 675,871 682,153 683,880 673,216 689,622 733,505 762,093      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 617,243 603,006 583,258 559,198 525,753 468,817 362,299 153,456      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 15,800                    
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 $ 401,789 401,170 392,447 383,246 376,096 337,256 330,753 331,362      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 349,406 329,021 306,270 260,764 242,696 162,469 96,025 36,381      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 26,447                    
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                    
Accident Year 2016 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 429,646 428,064 427,474 423,180 417,010 412,760 410,409 412,917      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 421,898 414,033 403,421 385,822 356,864 314,336 256,952   $ 173,668    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 2,931                    
Short-duration insurance contract, cumulative number of reported claims | claim 44,000                    
Accident Year 2016 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 722,138 722,551 723,925 722,697 722,967 723,989 729,675 730,239      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 725,516 723,111 717,788 710,624 707,472 695,566 667,750 429,132      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 2,262                    
Short-duration insurance contract, cumulative number of reported claims | claim 39,000                    
Accident Year 2016 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 344,072 334,003 320,518 304,542 285,053 263,327 245,631 255,303      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 269,239 233,096 198,517 169,379 143,838 86,509 54,395 25,295      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 19,683                    
Accident Year 2016 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 57,073 62,213 65,147 66,795 71,025 70,749 80,508 76,701      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 23,523 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 8,242                    
Accident Year 2016 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 215,714 216,854 215,723 215,295 221,413 222,463 223,315 227,950      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 212,577 211,837 209,232 206,018 202,436 194,150 162,814 $ 87,892      
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 347                    
Accident Year 2017 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,374,348 1,291,745 1,225,588 1,148,972 1,113,675 1,123,152 1,095,529        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,122,768 979,311 801,409 611,323 432,148 261,789 85,148        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 98,378                    
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 $ 708,336 715,018 721,170 724,056 715,055 724,697 778,964        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 660,237 642,292 613,675 574,889 508,546 397,464 171,006        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 14,769                    
Short-duration insurance contract, cumulative number of reported claims | claim 56,000                    
Accident Year 2017 | Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 393,029 397,102 381,613 358,676 332,201 320,784 333,254        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 315,361 283,349 243,813 198,351 154,953 99,433 28,072        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 59,723                    
Short-duration insurance contract, cumulative number of reported claims | claim 11,000                    
Accident Year 2017 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 513,458 511,283 502,221 474,469 459,826 443,203 423,120        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 493,942 479,144 445,527 397,517 337,599 272,020 173,779        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 5,358                    
Short-duration insurance contract, cumulative number of reported claims | claim 43,000                    
Accident Year 2017 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 718,987 723,401 724,399 726,956 728,426 731,643 741,002        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 715,238 712,526 707,471 708,449 691,499 645,228 401,547        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 4,803                    
Short-duration insurance contract, cumulative number of reported claims | claim 46,000                    
Accident Year 2017 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 327,489 310,129 282,547 270,699 254,696 235,054 245,491        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 245,037 207,435 163,823 128,691 93,211 52,668 18,921        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 26,167                    
Accident Year 2017 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 54,784 58,244 60,347 66,180 71,448 72,505 77,820        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 14,952 13,583 12,414 10,359 9,368 8,230 6,141        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 14,257                    
Accident Year 2017 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 117,569 118,633 120,254 121,985 120,284 128,611 125,574        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 114,035 113,418 112,061 103,833 98,416 80,976 $ 46,892        
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 918                    
Accident Year 2018 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,396,012 1,362,171 1,286,835 1,230,016 1,228,679 1,232,661          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,073,894 902,686 699,906 467,267 271,854 86,531          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 139,046                    
Short-duration insurance contract, cumulative number of reported claims | claim 30,000                    
Accident Year 2018 | Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 714,086 722,456 734,034 732,762 721,018 784,281          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 644,772 618,324 581,003 515,914 397,376 184,715          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 18,751                    
Short-duration insurance contract, cumulative number of reported claims | claim 54,000                    
Accident Year 2018 | Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 364,231 362,333 353,131 344,036 330,146 334,137          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 270,623 234,789 199,942 147,500 97,082 31,563          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 62,324                    
Short-duration insurance contract, cumulative number of reported claims | claim 12,000                    
Accident Year 2018 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 532,292 525,110 508,501 483,438 465,817 462,866          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 499,996 473,940 423,225 360,921 279,858 179,847          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 9,383                    
Short-duration insurance contract, cumulative number of reported claims | claim 43,000                    
Accident Year 2018 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 658,367 668,628 668,632 674,717 684,671 703,177          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 651,960 649,886 641,809 629,427 600,257 392,083          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 4,744                    
Short-duration insurance contract, cumulative number of reported claims | claim 41,000                    
Accident Year 2018 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 315,025 294,105 265,001 264,664 257,452 260,178          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 219,777 170,169 116,579 81,222 52,609 21,506          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 35,101                    
Accident Year 2018 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 70,929 73,571 76,478 76,242 77,482 78,929          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 21,094 18,836 15,436 12,728 10,884 6,241          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 13,613                    
Accident Year 2018 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 95,708 96,575 96,681 97,748 93,038 119,838          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 90,550 88,628 86,633 83,990 69,694 $ 35,563          
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 1,282                    
Accident Year 2019 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,187,560 1,157,392 1,148,852 1,204,248 1,330,660            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 812,453 614,351 417,860 222,274 70,540            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 181,892                    
Short-duration insurance contract, cumulative number of reported claims | claim 24,000                    
Accident Year 2019 | Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 652,424 668,222 704,008 716,430 725,245            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 579,332 548,585 485,203 380,454 172,478            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 12,026                    
Short-duration insurance contract, cumulative number of reported claims | claim 42,000                    
Accident Year 2019 | Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 299,806 312,169 336,231 373,550 391,532            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 196,332 168,670 128,579 79,968 27,895            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 56,121                    
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 $ 440,625 433,980 407,515 396,616 493,527            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 402,177 346,892 285,875 213,638 136,899            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 5,170                    
Short-duration insurance contract, cumulative number of reported claims | claim 29,000                    
Accident Year 2019 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 912,987 909,800 905,858 887,821 882,508            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 888,746 884,460 829,586 768,617 447,105            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 3,715                    
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 $ 345,540 346,355 327,257 329,319 333,743            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 217,958 171,527 109,396 66,937 28,291            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 62,760                    
Accident Year 2019 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 70,219 80,946 82,952 83,468 84,354            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 15,427 12,869 10,471 8,699 4,869            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 23,018                    
Accident Year 2019 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 136,155 137,504 132,670 133,897 131,649            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 118,503 111,508 102,593 81,238 $ 39,652            
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 844                    
Accident Year 2020 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,352,616 1,332,486 1,378,751 1,522,682              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 794,392 484,588 265,080 75,101              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 285,076                    
Short-duration insurance contract, cumulative number of reported claims | claim 26,000                    
Accident Year 2020 | Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 649,222 667,517 701,703 742,687              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 547,863 490,648 384,867 172,729              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 22,916                    
Short-duration insurance contract, cumulative number of reported claims | claim 46,000                    
Accident Year 2020 | Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 408,536 444,060 468,567 521,744              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 219,932 152,764 85,661 28,465              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 119,166                    
Short-duration insurance contract, cumulative number of reported claims | claim 12,000                    
Accident Year 2020 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 583,693 552,219 517,233 552,002              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 476,462 379,966 289,016 168,497              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 26,200                    
Short-duration insurance contract, cumulative number of reported claims | claim 35,000                    
Accident Year 2020 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 790,478 793,394 809,924 805,048              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 755,877 732,996 677,015 388,917              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 11,207                    
Short-duration insurance contract, cumulative number of reported claims | claim 33,000                    
Accident Year 2020 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 423,874 421,190 428,105 425,922              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 211,884 145,859 76,977 24,649              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 118,203                    
Accident Year 2020 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 84,681 89,220 90,980 98,110              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 11,412 8,872 6,026 4,586              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 27,325                    
Accident Year 2020 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 161,699 165,760 168,265 156,046              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 145,315 130,371 92,579 $ 31,624              
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 2,766                    
Accident Year 2021 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 1,798,403 1,815,653 1,810,560                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 704,307 352,886 92,248                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 688,730                    
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                    
Accident Year 2021 | Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 715,578 745,218 772,620                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 527,145 408,929 180,982                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 56,385                    
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 $ 553,174 582,764 644,950                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 160,965 90,416 33,336                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 307,294                    
Short-duration insurance contract, cumulative number of reported claims | claim 12,000                    
Accident Year 2021 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 763,806 740,688 723,266                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 531,324 394,136 237,885                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 83,225                    
Short-duration insurance contract, cumulative number of reported claims | claim 41,000                    
Accident Year 2021 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 880,040 889,173 904,554                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 827,552 761,924 440,856                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 25,611                    
Short-duration insurance contract, cumulative number of reported claims | claim 31,000                    
Accident Year 2021 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 474,466 501,517 488,982                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 159,100 83,590 28,029                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 208,624                    
Accident Year 2021 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 100,612 101,725 128,923                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 17,778 10,564 5,898                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 26,481                    
Accident Year 2021 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 202,600 205,569 205,886                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 152,174 128,640 $ 56,483                
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 24,379                    
Accident Year 2022 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 2,075,065 2,111,178                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 366,496 91,733                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 1,312,841                    
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 $ 758,657 784,906                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 418,788 195,204                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 146,880                    
Short-duration insurance contract, cumulative number of reported claims | claim 45,000                    
Accident Year 2022 | Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 634,269 642,582                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 121,991 40,973                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 404,820                    
Short-duration insurance contract, cumulative number of reported claims | claim 13,000                    
Accident Year 2022 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 907,503 890,682                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 467,227 271,552                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 226,458                    
Short-duration insurance contract, cumulative number of reported claims | claim 43,000                    
Accident Year 2022 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 997,176 1,035,378                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 846,828 531,838                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 66,732                    
Short-duration insurance contract, cumulative number of reported claims | claim 28,000                    
Accident Year 2022 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 466,192 472,810                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 66,568 17,622                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 305,158                    
Accident Year 2022 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 86,901 110,446                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 12,471 6,390                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 41,059                    
Accident Year 2022 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 172,995 176,457                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 110,095 $ 58,316                  
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 37,096                    
Accident Year 2023 | Other liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 2,379,621                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 95,137                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 2,070,647                    
Short-duration insurance contract, cumulative number of reported claims | claim 18,000                    
Accident Year 2023 | Workers' compensation                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 811,614                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 196,104                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 378,116                    
Short-duration insurance contract, cumulative number of reported claims | claim 46,000                    
Accident Year 2023 | Professional liability                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 648,064                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 40,200                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 499,645                    
Short-duration insurance contract, cumulative number of reported claims | claim 13,000                    
Accident Year 2023 | Auto                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 1,062,427                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 295,450                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 569,636                    
Short-duration insurance contract, cumulative number of reported claims | claim 41,000                    
Accident Year 2023 | Short-tail lines                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 1,254,576                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 559,186                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 450,792                    
Short-duration insurance contract, cumulative number of reported claims | claim 24,000                    
Accident Year 2023 | Casualty                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance $ 429,455                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 16,794                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 382,444                    
Accident Year 2023 | Monoline excess                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 120,046                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 5,723                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net 98,632                    
Accident Year 2023 | Property                      
Claims Development [Line Items]                      
Loss and loss expenses incurred, net of reinsurance 255,770                    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 69,379                    
Short-duration insurance contracts, incurred but not reported (IBNR) claims liability, net $ 137,881