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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, 2016
USD ($)
claim
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2011
USD ($)
Dec. 31, 2010
USD ($)
Dec. 31, 2009
USD ($)
Dec. 31, 2008
USD ($)
Dec. 31, 2007
USD ($)
Claims Development [Line Items]                    
Reserves for loss and loss adjustment expenses, net of reinsurance $ 10,229,797                  
Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 7,490,863                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,321,054                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 120,276                  
Reserves for loss and loss adjustment expenses, net of reinsurance 3,290,085                  
Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 5,117,528                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 3,528,447                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 138,281                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,727,362                  
Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,263,549                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 274,487                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 734,713                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,723,774                  
Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 2,018,235                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,288,059                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 7,173                  
Reserves for loss and loss adjustment expenses, net of reinsurance 737,349                  
Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 3,750,866                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 3,193,540                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 2,157                  
Reserves for loss and loss adjustment expenses, net of reinsurance 559,482                  
Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 5,306,798                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,701,669                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 3,099                  
Reserves for loss and loss adjustment expenses, net of reinsurance 608,228                  
Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 2,562,752                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 1,534,505                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 257,239                  
Reserves for loss and loss adjustment expenses, net of reinsurance 1,285,485                  
Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 681,850                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 542,644                  
Reserves for loss and loss adjustment expenses before 2007, net of reinsurance 557                  
Reserves for loss and loss adjustment expenses, net of reinsurance 139,763                  
Accident Year 2007 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 662,889 $ 663,122 $ 668,914 $ 693,669 $ 712,995 $ 726,338 $ 759,028 $ 799,566 $ 846,759 $ 888,917
Cumulative paid claims and claim adjustment expenses, net of reinsurance 609,568 589,270 564,049 528,583 477,050 414,621 332,348 233,194 126,938 46,993
IBNR $ 30,457                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2007 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 350,731 348,327 348,335 347,596 347,731 297,208 303,788 311,511 362,843 383,641
Cumulative paid claims and claim adjustment expenses, net of reinsurance 319,350 315,309 309,988 303,058 292,764 265,770 251,437 223,380 176,184 85,962
IBNR $ 10,478                  
Cumulative Number of Reported Claims | claim 48,000                  
Accident Year 2007 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 243,758 251,356 250,170 254,806 254,748 259,181 262,171 246,499 242,094 241,493
Cumulative paid claims and claim adjustment expenses, net of reinsurance 87,356 80,770 72,674 61,962 49,129 39,212 27,445 18,713 10,422 2,420
IBNR $ 40,607                  
Cumulative Number of Reported Claims | claim 2,000                  
Accident Year 2007 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 98,461 97,571 99,193 99,723 103,607 102,807 104,445 105,191 98,272 98,534
Cumulative paid claims and claim adjustment expenses, net of reinsurance 97,846 96,916 98,539 98,384 99,370 95,829 88,053 68,764 43,314 5,162
IBNR $ 595                  
Cumulative Number of Reported Claims | claim 2,000                  
Accident Year 2007 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 426,769 426,314 425,576 426,200 427,918 427,800 424,158 431,903 425,645 438,263
Cumulative paid claims and claim adjustment expenses, net of reinsurance 425,081 424,954 423,046 421,192 415,228 405,204 375,101 328,600 259,300 167,894
IBNR $ 307                  
Cumulative Number of Reported Claims | claim 49,000                  
Accident Year 2007 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 327,951 327,996 328,659 329,293 327,882 324,052 324,765 325,658 337,419 358,317
Cumulative paid claims and claim adjustment expenses, net of reinsurance 326,959 325,521 324,788 323,904 318,691 314,535 309,544 301,059 282,504 185,994
IBNR $ 991                  
Cumulative Number of Reported Claims | claim 21,000                  
Accident Year 2007 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 329,635 330,322 330,451 318,062 326,388 313,240 333,549 343,561 350,646 370,597
Cumulative paid claims and claim adjustment expenses, net of reinsurance 295,987 279,051 265,680 249,420 234,173 192,654 151,243 82,716 34,100 6,580
IBNR 9,329                  
Accident Year 2007 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 15,265 15,210 15,211 15,191 15,258 15,459 15,093 17,366 19,073 21,645
Cumulative paid claims and claim adjustment expenses, net of reinsurance 15,125 15,054 14,970 14,948 14,383 13,924 13,755 12,014 9,609 $ 2,387
IBNR (28)                  
Accident Year 2008 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 654,908 653,088 662,644 689,571 698,709 717,985 755,595 812,048 843,528  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 588,965 569,020 539,166 505,829 445,060 356,362 252,214 139,809 48,699  
IBNR $ 40,213                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2008 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 393,932 391,788 389,343 388,472 388,558 345,413 345,605 347,423 377,794  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 360,799 352,539 344,892 335,030 320,169 296,667 261,867 203,079 94,385  
IBNR $ 11,623                  
Cumulative Number of Reported Claims | claim 46,000                  
Accident Year 2008 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 183,802 194,302 193,395 195,385 197,321 202,419 211,624 240,528 243,067  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 48,042 39,257 34,418 25,063 14,489 9,907 6,302 2,942 2,464  
IBNR $ 52,994                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2008 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 108,507 109,242 107,320 107,760 110,268 111,452 116,539 119,953 113,171  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 106,381 105,299 100,352 97,796 96,585 85,588 66,167 37,818 9,998  
IBNR $ 1,857                  
Cumulative Number of Reported Claims | claim 2,000                  
Accident Year 2008 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 421,829 423,258 422,309 422,999 425,600 427,088 430,453 444,941 432,629  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 420,596 420,553 417,598 413,411 402,882 377,643 334,078 270,421 175,402  
IBNR $ 361                  
Cumulative Number of Reported Claims | claim 50,000                  
Accident Year 2008 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 392,782 392,457 393,137 393,913 393,943 396,055 402,911 415,554 428,243  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 390,626 388,068 386,792 385,350 379,494 380,158 369,446 353,632 248,653  
IBNR $ 1,202                  
Cumulative Number of Reported Claims | claim 23,000                  
Accident Year 2008 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 240,434 241,337 240,631 235,122 237,149 246,116 257,676 273,071 289,826  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 207,257 190,494 175,813 157,689 135,642 102,511 64,589 30,151 9,675  
IBNR 7,806                  
Accident Year 2008 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 19,061 18,968 18,952 19,498 18,879 19,449 19,597 21,055 23,896  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 18,940 18,839 18,816 18,591 18,111 17,670 16,620 13,993 $ 7,275  
IBNR 66                  
Accident Year 2009 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 558,566 563,317 567,578 595,643 605,497 632,324 664,619 699,630    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 489,453 474,893 432,622 388,508 314,994 217,471 124,901 45,461    
IBNR $ 41,517                  
Cumulative Number of Reported Claims | claim 23,000                  
Accident Year 2009 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 395,288 392,287 394,303 392,791 387,503 326,766 332,303 327,537    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 352,516 344,771 333,793 318,349 297,619 257,972 197,736 93,647    
IBNR $ 13,128                  
Cumulative Number of Reported Claims | claim 41,000                  
Accident Year 2009 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 189,646 193,697 190,439 202,239 210,273 220,124 214,506 228,830    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 45,963 39,200 32,419 25,925 18,338 12,444 8,893 5,298    
IBNR $ 54,310                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2009 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 153,574 150,875 150,545 148,108 148,992 145,638 139,340 134,978    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 144,792 144,024 138,947 127,798 117,738 85,929 52,597 12,608    
IBNR $ 2,523                  
Cumulative Number of Reported Claims | claim 3,000                  
Accident Year 2009 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 335,091 334,654 336,861 337,922 335,851 340,962 345,139 362,302    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 333,144 331,484 328,845 312,903 291,925 257,326 209,553 136,433    
IBNR $ 895                  
Cumulative Number of Reported Claims | claim 39,000                  
Accident Year 2009 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 327,199 327,465 328,711 332,621 332,782 344,157 354,134 368,106    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 323,248 320,701 318,775 318,138 313,052 311,568 296,125 214,062    
IBNR $ 1,129                  
Cumulative Number of Reported Claims | claim 19,000                  
Accident Year 2009 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 227,923 226,141 236,095 241,181 240,722 254,200 260,300 266,204    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 196,395 182,913 171,526 146,140 116,544 81,594 51,270 20,535    
IBNR 11,383                  
Accident Year 2009 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 23,246 23,704 23,824 24,071 24,160 25,424 25,118 26,917    
Cumulative paid claims and claim adjustment expenses, net of reinsurance 23,009 23,053 23,026 23,159 22,510 21,384 17,216 $ 8,256    
IBNR 60                  
Accident Year 2010 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 579,538 580,882 583,916 597,272 598,926 623,798 620,030      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 494,381 466,048 421,217 340,262 252,518 132,654 46,868      
IBNR $ 51,702                  
Cumulative Number of Reported Claims | claim 23,000                  
Accident Year 2010 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 429,762 429,952 426,622 420,604 411,527 361,808 358,734      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 374,013 362,078 344,631 320,154 281,280 214,034 107,742      
IBNR $ 23,233                  
Cumulative Number of Reported Claims | claim 42,000                  
Accident Year 2010 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 149,806 153,430 147,043 163,365 171,925 178,317 182,028      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 20,799 15,871 11,745 7,938 4,916 4,700 3,227      
IBNR $ 47,030                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2010 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 174,969 172,683 176,843 178,079 179,478 165,875 147,564      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 165,084 160,666 144,541 129,823 108,627 58,946 14,844      
IBNR $ 3,689                  
Cumulative Number of Reported Claims | claim 4,000                  
Accident Year 2010 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 330,586 331,865 333,028 329,109 330,432 320,302 310,591      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 326,804 324,997 313,262 295,355 263,639 208,790 136,029      
IBNR $ 1,193                  
Cumulative Number of Reported Claims | claim 38,000                  
Accident Year 2010 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 360,786 360,616 360,614 370,705 374,214 387,712 404,551      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 356,838 353,594 349,611 356,044 346,598 333,807 248,944      
IBNR $ 1,555                  
Cumulative Number of Reported Claims | claim 19,000                  
Accident Year 2010 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 196,105 200,288 213,384 223,537 228,052 236,246 236,460      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 155,603 140,946 122,709 101,312 72,274 41,463 16,049      
IBNR 18,259                  
Accident Year 2010 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 36,956 36,860 36,915 36,489 36,802 37,397 39,792      
Cumulative paid claims and claim adjustment expenses, net of reinsurance 36,285 35,764 35,442 34,632 33,656 29,231 $ 19,859      
IBNR 33                  
Accident Year 2011 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 655,443 660,412 665,885 666,887 681,815 676,275        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 527,270 475,650 384,107 271,011 146,070 50,702        
IBNR $ 65,108                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2011 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 474,076 472,087 470,026 457,134 444,887 419,364        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 408,304 385,759 355,909 309,509 236,207 106,157        
IBNR $ 29,589                  
Cumulative Number of Reported Claims | claim 43,000                  
Accident Year 2011 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 137,265 138,775 139,251 150,551 146,493 128,301        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 32,008 27,399 18,995 9,991 5,051 3,015        
IBNR $ 45,151                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2011 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 173,777 177,401 190,411 187,213 165,439 180,080        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 159,193 151,026 134,785 103,200 62,513 18,804        
IBNR $ 8,319                  
Cumulative Number of Reported Claims | claim 5,000                  
Accident Year 2011 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 341,200 343,701 335,024 330,125 322,724 314,038        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 333,987 321,814 296,370 262,685 211,756 135,350        
IBNR $ 2,567                  
Cumulative Number of Reported Claims | claim 38,000                  
Accident Year 2011 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 465,856 470,593 473,186 477,675 488,681 505,432        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 461,039 456,707 451,135 446,687 425,522 307,397        
IBNR $ 2,359                  
Cumulative Number of Reported Claims | claim 22,000                  
Accident Year 2011 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 244,833 247,739 241,309 243,468 247,663 239,562        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 175,933 152,163 123,450 88,490 47,500 15,670        
IBNR 27,815                  
Accident Year 2011 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 69,418 69,494 70,320 69,084 69,441 66,860        
Cumulative paid claims and claim adjustment expenses, net of reinsurance 67,940 67,301 66,012 63,560 51,216 $ 27,164        
IBNR 152                  
Accident Year 2012 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 723,961 716,617 711,727 712,889 704,519          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 517,386 422,269 304,171 162,543 59,669          
IBNR $ 93,444                  
Cumulative Number of Reported Claims | claim 25,000                  
Accident Year 2012 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 509,167 503,863 503,956 501,810 501,681          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 419,588 387,368 339,560 255,063 115,536          
IBNR $ 44,568                  
Cumulative Number of Reported Claims | claim 44,000                  
Accident Year 2012 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 115,583 117,066 112,477 101,663 98,799          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 28,055 24,120 19,184 7,421 715          
IBNR $ 34,805                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2012 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 238,086 250,074 263,640 240,210 236,681          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 189,796 159,061 127,980 86,356 21,524          
IBNR $ 23,185                  
Cumulative Number of Reported Claims | claim 8,000                  
Accident Year 2012 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 355,461 355,609 342,588 326,831 314,309          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 335,806 312,342 273,446 215,214 136,844          
IBNR $ 5,014                  
Cumulative Number of Reported Claims | claim 34,000                  
Accident Year 2012 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 546,259 550,656 556,418 560,110 555,079          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 532,900 524,678 514,064 462,315 284,916          
IBNR $ 8,173                  
Cumulative Number of Reported Claims | claim 41,000                  
Accident Year 2012 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 281,961 273,152 282,343 295,673 291,395          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 171,102 134,122 97,666 55,884 20,749          
IBNR 53,802                  
Accident Year 2012 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 64,170 65,526 64,989 69,515 73,551          
Cumulative paid claims and claim adjustment expenses, net of reinsurance 60,917 58,760 52,640 42,504 $ 11,908          
IBNR 545                  
Accident Year 2013 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 790,734 788,498 797,759 754,543            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 476,662 335,206 191,902 64,535            
IBNR $ 143,155                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2013 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 543,293 546,995 547,295 552,570            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 414,216 363,028 277,538 117,900            
IBNR $ 63,271                  
Cumulative Number of Reported Claims | claim 48,000                  
Accident Year 2013 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 46,028 50,448 54,171 75,214            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 3,013 2,159 679 279            
IBNR $ 28,016                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2013 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 247,687 242,639 245,925 266,538            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 176,103 119,553 63,927 23,550            
IBNR $ 36,873                  
Cumulative Number of Reported Claims | claim 8,000                  
Accident Year 2013 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 366,843 368,894 349,136 327,514            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 312,952 267,253 218,596 142,929            
IBNR $ 18,768                  
Cumulative Number of Reported Claims | claim 34,000                  
Accident Year 2013 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 588,399 588,285 598,549 588,182            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 564,813 542,173 492,780 316,170            
IBNR $ 10,793                  
Cumulative Number of Reported Claims | claim 51,000                  
Accident Year 2013 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 275,901 264,625 259,110 300,906            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 143,071 106,396 61,692 28,154            
IBNR 72,629                  
Accident Year 2013 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 99,513 100,897 97,153 125,131            
Cumulative paid claims and claim adjustment expenses, net of reinsurance 91,192 85,182 69,056 $ 34,625            
IBNR 1,312                  
Accident Year 2014 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 851,724 850,666 850,243              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 342,933 192,893 79,801              
IBNR $ 282,056                  
Cumulative Number of Reported Claims | claim 26,000                  
Accident Year 2014 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 627,862 637,307 639,436              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 412,716 319,743 148,405              
IBNR $ 110,364                  
Cumulative Number of Reported Claims | claim 51,000                  
Accident Year 2014 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 59,903 66,854 68,521              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 4,266 2,277 377              
IBNR $ 37,366                  
Cumulative Number of Reported Claims | claim 1,000                  
Accident Year 2014 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 255,700 246,068 252,167              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 134,726 83,672 19,391              
IBNR $ 75,613                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2014 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 394,998 385,345 363,968              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 306,594 237,766 155,615              
IBNR $ 28,672                  
Cumulative Number of Reported Claims | claim 36,000                  
Accident Year 2014 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 705,706 718,422 710,961              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 652,405 601,225 374,214              
IBNR $ 22,492                  
Cumulative Number of Reported Claims | claim 60,000                  
Accident Year 2014 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 306,791 307,849 311,776              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 114,334 66,656 20,394              
IBNR 119,679                  
Accident Year 2014 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 88,592 86,848 102,644              
Cumulative paid claims and claim adjustment expenses, net of reinsurance 74,695 61,966 $ 36,723              
IBNR 1,552                  
Accident Year 2015 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 992,128 953,822                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 208,837 83,378                
IBNR $ 546,566                  
Cumulative Number of Reported Claims | claim 24,000                  
Accident Year 2015 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 690,656 712,800                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 323,879 139,320                
IBNR $ 214,854                  
Cumulative Number of Reported Claims | claim 52,000                  
Accident Year 2015 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 61,574 74,777                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 2,484 2,069                
IBNR $ 45,861                  
Cumulative Number of Reported Claims | claim 0                  
Accident Year 2015 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 256,432 259,368                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 85,348 20,496                
IBNR $ 121,119                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2015 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 390,590 389,914                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 242,031 160,239                
IBNR $ 58,001                  
Cumulative Number of Reported Claims | claim 39,000                  
Accident Year 2015 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 774,801 752,486                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 645,601 397,084                
IBNR $ 56,787                  
Cumulative Number of Reported Claims | claim 62,000                  
Accident Year 2015 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 224,777 250,976                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 51,082 17,259                
IBNR 115,419                  
Accident Year 2015 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 106,965 115,942                
Cumulative paid claims and claim adjustment expenses, net of reinsurance 81,565 $ 51,982                
IBNR 5,887                  
Accident Year 2016 | Other liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 1,020,972                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 65,599                  
IBNR $ 770,954                  
Cumulative Number of Reported Claims | claim 18,000                  
Accident Year 2016 | Primary workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 702,761                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 143,066                  
IBNR $ 339,257                  
Cumulative Number of Reported Claims | claim 49,000                  
Accident Year 2016 | Excess workers' compensation                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 76,184                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 2,501                  
IBNR $ 62,033                  
Cumulative Number of Reported Claims | claim 0                  
Accident Year 2016 | Professional liability                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 311,042                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 28,789                  
IBNR $ 232,577                  
Cumulative Number of Reported Claims | claim 9,000                  
Accident Year 2016 | Commercial automobile                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 387,499                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 156,545                  
IBNR $ 124,075                  
Cumulative Number of Reported Claims | claim 35,000                  
Accident Year 2016 | Short-tail lines                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 817,059                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 447,240                  
IBNR $ 179,583                  
Cumulative Number of Reported Claims | claim 48,000                  
Accident Year 2016 | Casualty                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance $ 234,392                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 23,741                  
IBNR 173,456                  
Accident Year 2016 | Property                    
Claims Development [Line Items]                    
Loss and loss expenses incurred, net of reinsurance 158,664                  
Cumulative paid claims and claim adjustment expenses, net of reinsurance 72,976                  
IBNR $ 43,516