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Summary of Information about Incurred Claims Development and Cumulative Paid Claims and Allocated Loss Adjustment Expense (ALAE), Net of Reinsurance (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Claim
Dec. 31, 2016
USD ($)
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 ($)
Dec. 31, 2008
USD ($)
Claims Development [Line Items]                    
Total Net reserves $ 1,640,874                  
U.S.Insurance | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 560,568                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 474,664                  
Net reserves lesser than 2008 15,187                  
Total Net reserves $ 101,091                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 27.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 31.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 16.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 8.40%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 7.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 3.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 2.20%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 1.90%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 1.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 0.60%                  
U.S.Insurance | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 1,683,912                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 859,169                  
Net reserves lesser than 2008 24,266                  
Total Net reserves $ 849,009                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 5.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 10.90%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 17.10%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 18.60%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 15.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 10.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 5.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 5.50%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 5.10%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 2.60%                  
U.S.Insurance | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 547,105                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 439,684                  
Net reserves lesser than 2008 881                  
Total Net reserves $ 108,302                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 5.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 30.60%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 25.90%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 13.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 9.20%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 8.90%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 3.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 1.40%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 5.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 0.00%                  
U.S.Insurance | 2008 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 66,849 $ 66,937 $ 66,717 $ 68,557 $ 68,393 $ 73,494 $ 73,734 $ 72,259 $ 67,421 $ 56,878
Total IBNR $ 282                  
Number of Reported Claims | Claim 3,194                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 64,433 64,029 63,140 63,027 62,700 62,398 56,460 54,175 36,397 7,554
U.S.Insurance | 2008 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 154,169 152,814 147,763 142,641 138,499 136,498 135,255 138,860 152,659 151,401
Total IBNR $ 3,706                  
Number of Reported Claims | Claim 10,937                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 146,295 142,284 128,886 119,766 111,420 99,825 78,024 54,663 32,307 10,266
U.S.Insurance | 2008 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 43,003 41,766 37,620 37,497 37,939 37,603 35,844 39,130 38,449 34,346
Total IBNR $ 875                  
Number of Reported Claims | Claim 770                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 42,046 42,056 37,175 35,598 35,206 34,902 30,581 25,042 15,288 $ 1,880
U.S.Insurance | 2009 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 51,537 50,874 51,783 49,571 48,366 50,501 56,052 59,278 62,556  
Total IBNR $ 205                  
Number of Reported Claims | Claim 2,323                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 50,191 49,573 46,894 46,033 43,450 41,650 35,449 29,266 12,947  
U.S.Insurance | 2009 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 149,622 150,294 147,036 146,321 141,186 135,984 130,860 129,613 127,237  
Total IBNR $ 6,143                  
Number of Reported Claims | Claim 8,515                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 138,829 136,636 129,461 123,981 110,648 86,736 55,773 33,049 11,245  
U.S.Insurance | 2009 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 61,083 58,143 58,660 58,487 60,789 58,600 61,023 53,273 47,473  
Total IBNR $ 788                  
Number of Reported Claims | Claim 1,370                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 58,150 58,095 57,744 53,859 55,378 51,642 41,675 21,742 $ 3,837  
U.S.Insurance | 2010 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 56,281 56,458 57,008 52,847 54,813 60,035 63,812 64,753    
Total IBNR $ (224)                  
Number of Reported Claims | Claim 2,007                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 55,249 55,096 51,887 48,114 42,896 39,825 32,323 16,582    
U.S.Insurance | 2010 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 110,170 107,914 104,955 103,038 100,006 94,317 96,446 96,885    
Total IBNR $ 7,776                  
Number of Reported Claims | Claim 7,874                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 97,125 90,625 85,740 77,302 65,094 46,478 26,482 11,985    
U.S.Insurance | 2010 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 86,164 85,348 86,787 74,535 75,619 68,375 64,020 52,302    
Total IBNR $ 932                  
Number of Reported Claims | Claim 1,431                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 84,529 84,511 82,013 59,765 54,019 46,868 23,147 $ 2,981    
U.S.Insurance | 2011 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 59,750 60,329 61,912 63,099 67,640 69,293 70,219      
Total IBNR $ 630                  
Number of Reported Claims | Claim 1,910                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 58,558 57,929 56,955 52,408 48,850 39,821 14,955      
U.S.Insurance | 2011 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 100,461 98,238 90,613 89,666 86,557 78,093 79,832      
Total IBNR $ 12,051                  
Number of Reported Claims | Claim 6,450                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 79,682 70,486 58,489 46,638 30,359 14,579 3,630      
U.S.Insurance | 2011 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 82,558 79,136 79,181 73,028 70,240 62,087 58,116      
Total IBNR $ 699                  
Number of Reported Claims | Claim 1,726                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 78,258 75,641 65,710 59,576 43,551 23,878 $ 3,204      
U.S.Insurance | 2012 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 70,887 70,337 68,841 72,219 87,606 85,275        
Total IBNR $ (619)                  
Number of Reported Claims | Claim 1,856                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 69,059 67,220 60,734 54,673 46,308 27,254        
U.S.Insurance | 2012 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 111,146 107,245 99,207 91,502 89,758 103,089        
Total IBNR $ 18,194                  
Number of Reported Claims | Claim 4,579                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 82,906 67,502 47,005 27,513 17,311 4,761        
U.S.Insurance | 2012 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 58,563 58,344 55,917 57,097 54,480 61,196        
Total IBNR $ 2,254                  
Number of Reported Claims | Claim 2,175                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 54,728 49,675 44,028 35,681 22,635 3,855        
U.S.Insurance | 2013 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 56,319 54,381 51,748 57,865 61,356          
Total IBNR $ 144                  
Number of Reported Claims | Claim 1,814                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 51,466 47,162 41,488 35,471 17,831          
U.S.Insurance | 2013 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 149,409 137,298 129,100 125,952 139,517          
Total IBNR $ 20,569                  
Number of Reported Claims | Claim 3,898                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 118,506 84,494 48,716 20,914 7,146          
U.S.Insurance | 2013 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 54,228 48,298 49,104 54,982 60,899          
Total IBNR $ 3,915                  
Number of Reported Claims | Claim 1,982                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 47,062 38,876 32,904 22,167 4,677          
U.S.Insurance | 2014 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 45,762 41,006 42,684 59,270            
Total IBNR $ (377)                  
Number of Reported Claims | Claim 1,927                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 41,021 34,899 23,415 10,717            
U.S.Insurance | 2014 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 155,248 148,541 159,477 191,515            
Total IBNR $ 51,231                  
Number of Reported Claims | Claim 3,880                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 74,979 44,468 13,264 5,229            
U.S.Insurance | 2014 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 42,060 39,826 35,870 50,020            
Total IBNR $ 5,255                  
Number of Reported Claims | Claim 2,017                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 35,114 29,392 14,371 2,304            
U.S.Insurance | 2015 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 50,331 51,864 61,106              
Total IBNR $ 5,148                  
Number of Reported Claims | Claim 2,187                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 40,586 31,010 16,945              
U.S.Insurance | 2015 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 214,547 218,973 244,311              
Total IBNR $ 76,700                  
Number of Reported Claims | Claim 3,672                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 78,856 24,057 3,575              
U.S.Insurance | 2015 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 31,088 30,764 31,659              
Total IBNR $ 8,721                  
Number of Reported Claims | Claim 1,674                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 18,428 11,249 2,257              
U.S.Insurance | 2016 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 48,791 53,011                
Total IBNR $ 12,748                  
Number of Reported Claims | Claim 2,352                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 25,154 11,610                
U.S.Insurance | 2016 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 243,754 271,351                
Total IBNR $ 190,052                  
Number of Reported Claims | Claim 3,136                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 29,131 5,831                
U.S.Insurance | 2016 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 36,650 41,673                
Total IBNR $ 12,608                  
Number of Reported Claims | Claim 1,806                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 18,687 2,382                
U.S.Insurance | 2017 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 54,061                  
Total IBNR $ 23,327                  
Number of Reported Claims | Claim 1,986                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 18,947                  
U.S.Insurance | 2017 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 295,386                  
Total IBNR $ 263,648                  
Number of Reported Claims | Claim 2,527                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 12,860                  
U.S.Insurance | 2017 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 51,708                  
Total IBNR $ 44,396                  
Number of Reported Claims | Claim 1,902                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 2,682                  
International Insurance | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 945,486                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 734,899                  
Net reserves lesser than 2008 11,072                  
Total Net reserves $ 221,659                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 25.10%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 29.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 19.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 7.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 4.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 2.40%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 2.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 1.90%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 1.50%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 0.00%                  
International Insurance | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 441,670                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 346,815                  
Net reserves lesser than 2008 825                  
Total Net reserves $ 95,680                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 25.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 32.40%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 26.50%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 13.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 4.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 (1.00%)                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 (1.50%)                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 0.90%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 (0.10%)                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 0.20%                  
International Insurance | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 232,082                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 115,573                  
Net reserves lesser than 2008 274                  
Total Net reserves $ 116,783                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 6.70%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 8.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 16.60%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 20.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 15.20%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 5.40%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 35.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 0.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 2.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 0.20%                  
International Insurance | 2008 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 83,962 84,449 84,122 80,589 80,793 85,148        
Total IBNR $ (135)                  
Number of Reported Claims | Claim 5,733                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 80,183 80,185 79,024 75,255 73,799 71,355        
International Insurance | 2008 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 41,472 41,536 33,877 34,495 34,428 37,256        
Total IBNR $ 67                  
Number of Reported Claims | Claim 913                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 40,333 40,249 40,398 39,869 39,489 38,275        
International Insurance | 2008 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 14,463 14,082 13,829 13,798 13,524 11,749        
Total IBNR $ 258                  
Number of Reported Claims | Claim 439                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 14,105 14,071 13,784 13,754 13,360 12,198        
International Insurance | 2009 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 81,729 82,074 83,477 90,527 91,705 96,465        
Total IBNR $ (265)                  
Number of Reported Claims | Claim 5,045                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 78,772 77,487 76,783 73,321 70,553 66,388        
International Insurance | 2009 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 34,461 34,186 40,692 40,984 40,896 38,680        
Total IBNR $ 206                  
Number of Reported Claims | Claim 885                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 33,494 33,405 33,317 33,685 33,223 28,919        
International Insurance | 2009 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 15,988 16,020 17,016 17,365 17,225 16,661        
Total IBNR $ 758                  
Number of Reported Claims | Claim 624                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 14,918 14,331 14,008 12,983 10,901 8,954        
International Insurance | 2010 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 108,523 112,259 114,940 119,156 130,102 132,080        
Total IBNR $ (428)                  
Number of Reported Claims | Claim 5,129                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 103,646 103,188 100,897 96,920 90,871 78,509        
International Insurance | 2010 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 42,613 42,822 42,196 41,922 42,527 43,096        
Total IBNR $ 309                  
Number of Reported Claims | Claim 1,089                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 41,691 41,204 41,102 43,429 40,880 35,192        
International Insurance | 2010 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 14,639 15,864 14,221 14,225 14,023 14,423        
Total IBNR $ 247                  
Number of Reported Claims | Claim 745                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 13,643 13,597 12,840 10,659 7,320 5,909        
International Insurance | 2011 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 74,309 73,970 77,848 82,441 85,720 100,162        
Total IBNR $ (500)                  
Number of Reported Claims | Claim 4,643                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 70,022 69,672 68,702 63,778 59,341 45,896        
International Insurance | 2011 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 55,127 59,516 59,529 59,405 61,564 71,531        
Total IBNR $ 221                  
Number of Reported Claims | Claim 1,216                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 52,225 54,239 53,318 50,753 47,851 38,548        
International Insurance | 2011 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 12,823 6,989 8,923 19,429 18,125 15,390        
Total IBNR $ 1,769                  
Number of Reported Claims | Claim 665                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 16,642 4,677 4,087 2,994 2,038 1,221        
International Insurance | 2012 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 92,983 93,052 100,029 107,636 132,245 110,316        
Total IBNR $ (1,224)                  
Number of Reported Claims | Claim 4,908                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 83,381 81,411 78,789 70,657 54,954 23,382        
International Insurance | 2012 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 33,075 32,622 32,472 36,444 37,520 45,693        
Total IBNR $ 152                  
Number of Reported Claims | Claim 1,358                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 31,596 31,332 30,135 27,084 21,578 13,238        
International Insurance | 2012 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 18,068 17,168 13,457 12,789 11,506 6,835        
Total IBNR $ 2,365                  
Number of Reported Claims | Claim 1,102                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 6,933 7,526 5,600 4,293 3,236 2,141        
International Insurance | 2013 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 84,100 81,989 82,641 94,614 87,987          
Total IBNR $ 3,168                  
Number of Reported Claims | Claim 4,716                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 67,730 64,859 58,687 46,552 23,731          
International Insurance | 2013 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 35,443 34,499 36,599 37,520 36,044          
Total IBNR $ 1,027                  
Number of Reported Claims | Claim 1,592                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 30,279 28,682 25,840 18,441 7,160          
International Insurance | 2013 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 16,842 15,748 14,532 4,099 10,218          
Total IBNR $ 1,607                  
Number of Reported Claims | Claim 1,174                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 13,906 9,478 3,078 2,285 733          
International Insurance | 2014 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 119,767 116,899 117,974 104,397            
Total IBNR $ (1,483)                  
Number of Reported Claims | Claim 4,986                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 90,619 81,941 57,629 29,977            
International Insurance | 2014 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 16,449 20,997 22,787 27,829            
Total IBNR $ (1,396)                  
Number of Reported Claims | Claim 1,300                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 16,805 13,232 9,015 3,902            
International Insurance | 2014 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 21,758 23,861 24,519 20,662            
Total IBNR $ 4,216                  
Number of Reported Claims | Claim 1,747                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 8,945 5,228 2,493 1,387            
International Insurance | 2015 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 96,614 95,001 99,067              
Total IBNR $ (1,913)                  
Number of Reported Claims | Claim 7,400                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 76,044 54,550 22,862              
International Insurance | 2015 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 33,969 36,689 34,889              
Total IBNR $ 10                  
Number of Reported Claims | Claim 1,986                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 29,624 18,436 6,845              
International Insurance | 2015 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 35,586 30,539 28,319              
Total IBNR $ 9,550                  
Number of Reported Claims | Claim 2,370                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 17,502 5,939 3,457              
International Insurance | 2016 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 95,584 88,388                
Total IBNR $ 8,335                  
Number of Reported Claims | Claim 9,114                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 55,420 23,665                
International Insurance | 2016 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 92,108 79,393                
Total IBNR $ 18,665                  
Number of Reported Claims | Claim 3,556                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 55,749 26,190                
International Insurance | 2016 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 34,218 36,167                
Total IBNR $ 22,567                  
Number of Reported Claims | Claim 2,744                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 6,955 2,076                
International Insurance | 2017 | Marine                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 107,915                  
Total IBNR $ 32,198                  
Number of Reported Claims | Claim 7,385                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 29,082                  
International Insurance | 2017 | Property Casualty                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 56,953                  
Total IBNR $ 10,229                  
Number of Reported Claims | Claim 2,213                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 15,019                  
International Insurance | 2017 | Professional Liability                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 47,697                  
Total IBNR $ 41,954                  
Number of Reported Claims | Claim 1,522                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 2,024                  
Global Reinsurance                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 765,010                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance 619,164                  
Net reserves lesser than 2008 2,504                  
Total Net reserves $ 148,350                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 1 39.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 2 34.00%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 3 3.20%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 4 3.80%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 5 8.20%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 6 3.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 7 3.30%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 8 0.50%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 9 0.20%                  
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance 10 0.70%                  
Global Reinsurance | 2008                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance $ 13,123 13,487 13,792 14,198 14,846 14,467        
Total IBNR $ 529                  
Number of Reported Claims | Claim 306                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 11,797 11,702 11,640 11,661 11,799 11,775        
Global Reinsurance | 2009                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 10,406 10,567 10,516 11,267 11,381 11,247        
Total IBNR $ 53                  
Number of Reported Claims | Claim 200                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 10,133 10,133 10,018 9,060 8,930 8,694        
Global Reinsurance | 2010                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 13,159 12,631 11,908 14,142 13,110 13,150        
Total IBNR $ (54)                  
Number of Reported Claims | Claim 201                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 11,272 11,207 11,154 10,529 10,349 10,005        
Global Reinsurance | 2011                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 73,530 72,877 73,610 76,164 76,362 53,868        
Total IBNR $ (438)                  
Number of Reported Claims | Claim 769                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 72,414 72,246 72,102 71,816 71,138 43,450        
Global Reinsurance | 2012                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 119,771 115,058 88,367 92,730 99,116 107,189        
Total IBNR $ (815)                  
Number of Reported Claims | Claim 1,456                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 117,782 111,962 84,801 83,408 77,227 $ 37,746        
Global Reinsurance | 2013                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 102,433 105,172 102,632 118,779 114,873          
Total IBNR $ 1,067                  
Number of Reported Claims | Claim 1,285                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 100,059 98,484 90,721 82,701 $ 53,476          
Global Reinsurance | 2014                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 118,255 121,716 142,058 119,379            
Total IBNR $ 5,062                  
Number of Reported Claims | Claim 1,070                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 108,794 105,648 119,249 $ 66,286            
Global Reinsurance | 2015                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 94,428 91,767 101,764              
Total IBNR $ 6,897                  
Number of Reported Claims | Claim 896                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 76,973 64,527 $ 41,660              
Global Reinsurance | 2016                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 90,755 88,543                
Total IBNR $ 15,972                  
Number of Reported Claims | Claim 749                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 66,277 $ 36,352                
Global Reinsurance | 2017                    
Claims Development [Line Items]                    
Loss and ALAE, Net of Reinsurance 129,150                  
Total IBNR $ 60,576                  
Number of Reported Claims | Claim 428                  
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance $ 43,663