Schedule III. Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	$ 663,991	$ 476,661	$ 426,551
Future Policy Benefits, Losses, Claims and Loss Expenses	9,384,349	6,076,271	5,080,408
Unearned Premiums	2,530,975	1,716,021	1,477,609
Premium Revenue	3,338,403	1,976,129	1,717,575
Net Investment Income	423,833	261,866	222,209
Benefits, Claims, Losses and Settlement Expenses	2,097,021	1,120,018	1,861,428
Amortization of Deferred Policy Acquisition Costs	762,232	432,989	346,892
Other Operating Expenses	222,733	178,267	160,778
Net Written Premiums	3,381,493	2,131,902	1,871,325
Property
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	79,795	66,656	63,583
Future Policy Benefits, Losses, Claims and Loss Expenses	4,073,850	3,086,254	2,486,390
Unearned Premiums	539,183	379,943	347,032
Premium Revenue	1,627,494	1,050,831	931,070
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	965,424	497,895	1,297,985
Amortization of Deferred Policy Acquisition Costs	313,761	177,912	113,816
Other Operating Expenses	139,015	112,954	94,194
Net Written Premiums	1,654,259	1,055,188	978,014
Casualty and Specialty
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	584,196	410,005	362,968
Future Policy Benefits, Losses, Claims and Loss Expenses	5,310,059	2,985,393	2,575,492
Unearned Premiums	1,991,792	1,336,078	1,130,577
Premium Revenue	1,710,909	925,298	786,501
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	1,131,637	622,320	565,026
Amortization of Deferred Policy Acquisition Costs	448,678	255,079	233,077
Other Operating Expenses	84,546	64,883	66,548
Net Written Premiums	1,727,234	1,076,714	893,307
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	0	0	0
Future Policy Benefits, Losses, Claims and Loss Expenses	440	4,624	18,526
Unearned Premiums	0	0	0
Premium Revenue	0	0	4
Net Investment Income	423,833	261,866	222,209
Benefits, Claims, Losses and Settlement Expenses	(40)	(197)	(1,583)
Amortization of Deferred Policy Acquisition Costs	(207)	(2)	(1)
Other Operating Expenses	(828)	430	36
Net Written Premiums	$ 0	$ 0	$ 4