Schedule III. Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	$ 335,325	$ 199,380	$ 110,059
Future Policy Benefits, Losses, Claims and Loss Expenses	2,848,294	2,767,045	1,412,510
Unearned Premiums	1,231,573	889,102	512,386
Premium Revenue	1,403,430	1,400,551	1,062,416
Net Investment Income	181,726	152,567	124,316
Benefits, Claims, Losses and Settlement Expenses	530,831	448,238	197,947
Amortization of Deferred Policy Acquisition Costs	289,323	238,592	144,476
Other Operating Expenses	197,749	219,112	190,639
Net Written Premiums	1,535,312	1,416,183	1,068,236
Property
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	46,938	39,763	41,161
Future Policy Benefits, Losses, Claims and Loss Expenses	627,774	706,199	634,725
Unearned Premiums	289,080	272,050	280,238
Premium Revenue	720,951	805,985	698,416
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	151,545	128,290	16,643
Amortization of Deferred Policy Acquisition Costs	97,594	94,249	66,262
Other Operating Expenses	108,642	118,666	117,943
Net Written Premiums	725,321	726,145	662,552
Casualty and Specialty
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	288,387	159,617	68,898
Future Policy Benefits, Losses, Claims and Loss Expenses	2,195,126	2,033,168	736,099
Unearned Premiums	942,493	617,052	232,148
Premium Revenue	682,337	594,614	363,632
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	380,396	320,818	187,441
Amortization of Deferred Policy Acquisition Costs	191,729	144,095	84,762
Other Operating Expenses	88,984	100,180	72,393
Net Written Premiums	809,848	690,086	405,340
Other
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	0	0	0
Future Policy Benefits, Losses, Claims and Loss Expenses	25,394	27,678	41,686
Unearned Premiums	0	0	0
Premium Revenue	142	(48)	368
Net Investment Income	181,726	152,567	124,316
Benefits, Claims, Losses and Settlement Expenses	(1,110)	(870)	(6,137)
Amortization of Deferred Policy Acquisition Costs	0	248	(6,548)
Other Operating Expenses	123	266	303
Net Written Premiums	$ 143	$ (48)	$ 344