Schedule III. Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	$ 426,551	$ 335,325	$ 199,380
Future Policy Benefits, Losses, Claims and Loss Expenses	5,080,408	2,848,294	2,767,045
Unearned Premiums	1,477,609	1,231,573	889,102
Premium Revenue	1,717,575	1,403,430	1,400,551
Net Investment Income	222,209	181,726	152,567
Benefits, Claims, Losses and Settlement Expenses	1,861,428	530,831	448,238
Amortization of Deferred Policy Acquisition Costs	346,892	289,323	238,592
Other Operating Expenses	160,778	197,749	219,112
Net Written Premiums	1,871,325	1,535,312	1,416,183
Property
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	63,583	46,938	39,763
Future Policy Benefits, Losses, Claims and Loss Expenses	2,486,390	627,774	706,199
Unearned Premiums	347,032	289,080	272,050
Premium Revenue	931,070	720,951	805,985
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	1,297,985	151,545	128,290
Amortization of Deferred Policy Acquisition Costs	113,816	97,594	94,249
Other Operating Expenses	94,194	108,642	118,666
Net Written Premiums	978,014	725,321	726,145
Casualty and Specialty
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	362,968	288,387	159,617
Future Policy Benefits, Losses, Claims and Loss Expenses	2,575,492	2,195,126	2,033,168
Unearned Premiums	1,130,577	942,493	617,052
Premium Revenue	786,501	682,337	594,614
Net Investment Income	0	0	0
Benefits, Claims, Losses and Settlement Expenses	565,026	380,396	320,818
Amortization of Deferred Policy Acquisition Costs	233,077	191,729	144,095
Other Operating Expenses	66,548	88,984	100,180
Net Written Premiums	893,307	809,848	690,086
Other
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	0	0	0
Future Policy Benefits, Losses, Claims and Loss Expenses	18,526	25,394	27,678
Unearned Premiums	0	0	0
Premium Revenue	4	142	(48)
Net Investment Income	222,209	181,726	152,567
Benefits, Claims, Losses and Settlement Expenses	(1,583)	(1,110)	(870)
Amortization of Deferred Policy Acquisition Costs	(1)	0	248
Other Operating Expenses	36	123	266
Net Written Premiums	$ 4	$ 143	$ (48)