Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	$ 581,863	$ 511,573	$ 411,587
Reverse for Losses and Loss Adjustment Expenses	13,611,313	13,119,090	11,884,321
Unearned Premium Reserves	3,056,735	2,517,612	2,000,556
Premiums Earned	7,403,686	6,931,699	5,937,840
Net Investment Income	647,139
Incurred Loss and Loss Adjustment Expenses	4,922,898	5,651,403	4,522,581
Amortization of Deferred Acquisition Costs	1,703,726	1,519,030	1,303,963
Other Operating Expenses	440,899	371,541	318,817
Net Written Premium	7,824,424	7,414,428	6,244,661
Domestic Country [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	358,108	334,818	289,636
Reverse for Losses and Loss Adjustment Expenses	7,823,095	8,154,003	7,254,043
Unearned Premium Reserves	2,014,712	1,695,208	1,460,291
Premiums Earned	4,383,837	4,167,619	3,642,547
Net Investment Income	320,923
Incurred Loss and Loss Adjustment Expenses	2,761,871	3,849,982	2,727,649
Amortization of Deferred Acquisition Costs	1,007,282	835,823	712,568
Other Operating Expenses	348,861	288,933	241,962
Net Written Premium	4,605,731	4,350,157	3,875,982
International Country [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	66,183	54,253	53,186
Reverse for Losses and Loss Adjustment Expenses	2,655,458	2,209,202	2,175,500
Unearned Premium Reserves	344,222	261,611	253,626
Premiums Earned	1,475,669	1,439,882	1,202,043
Net Investment Income	37,297
Incurred Loss and Loss Adjustment Expenses	1,193,816	992,704	1,059,640
Amortization of Deferred Acquisition Costs	342,400	364,010	287,688
Other Operating Expenses	41,690	39,042	38,844
Net Written Premium	1,551,753	1,458,745	1,229,597
Bermuda Country [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	157,572	122,502	68,765
Reverse for Losses and Loss Adjustment Expenses	3,132,760	2,755,885	2,454,778
Unearned Premium Reserves	697,801	560,793	286,639
Premiums Earned	1,544,180	1,324,198	1,093,250
Net Investment Income	288,919
Incurred Loss and Loss Adjustment Expenses	967,211	808,717	735,292
Amortization of Deferred Acquisition Costs	354,044	319,197	303,707
Other Operating Expenses	50,348	43,566	38,011
Net Written Premium	$ 1,666,940	$ 1,605,526	$ 1,139,082