Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 235,053	$ 237,376	$ 231,456	$ 221,341	$ 226,192	$ 239,234	$ 213,508	$ 217,288	$ 925,226	$ 896,222	$ 844,371
Investment income, net of expenses	28,094	27,515	27,248	27,809	27,926	25,939	25,756	24,120	110,666	103,741	87,647
Realized (losses) gains	(52)	5,092	836	3,056	1,228	640	166	26,327	8,932	28,361	1,357
Other revenue	3,425	3,867	3,994	6,373	3,087	3,698	3,699	2,480	17,659	12,964	9,259
Loss incurred, net	47,658	60,897	46,590	85,012	95,066	76,458	90,238	81,785	240,157	343,547	496,077
Underwriting and other expenses, net	56,824	53,981	49,837	56,439	53,858	65,805	37,915	51,969	217,081	209,547
Loss on debt extinguishment	0	75,223	1,868	13,440	507	0	0	0	90,531	507	837
Provision for income tax	54,551	27,131	56,018	34,497	6,584	(695,604)	1,322	3,385	172,197	(684,313)	2,774
Net income	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 102,418	$ 822,852	$ 113,654	$ 133,076	$ 342,517	$ 1,172,000	$ 251,949
Earnings per Share [Abstract]
Basic (in dollars per share)	$ 0.31	$ 0.16	$ 0.32	$ 0.20	$ 0.30	$ 2.42	$ 0.33	$ 0.39	$ 1.00	$ 3.45	$ 0.74
Diluted (in dollars per share)	$ 0.28	$ 0.14	$ 0.26	$ 0.17	$ 0.24	$ 1.78	$ 0.28	$ 0.32	$ 0.86	$ 2.60	$ 0.64