Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 237,425	$ 237,083	$ 231,136	$ 229,103	$ 235,053	$ 237,376	$ 231,456	$ 221,341	$ 934,747	$ 925,226	$ 896,222
Investment income, net of expenses	31,276	30,402	29,716	29,477	28,094	27,515	27,248	27,809	120,871	110,666	103,741
Realized gains (losses)	460	(47)	(42)	(122)	(52)	5,092	836	3,056	249	8,932	28,361
Other revenue	2,341	2,922	2,502	2,422	3,425	3,867	3,994	6,373	10,187	17,659	12,964
Loss incurred, net	(30,996)	29,747	27,339	27,619	47,658	60,897	46,590	85,012	53,709	240,157	343,547
Underwriting and other expenses, net	57,042	56,146	55,292	59,304	56,824	53,981	49,837	56,439	227,784	217,081
Loss on debt extinguishment	0	0	65	0	0	75,223	1,868	13,440	65	90,531	507
Provision for income tax	218,142	64,440	61,994	84,159	54,551	27,131	56,018	34,497	428,735	172,197	(684,313)
Net income	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 107,487	$ 56,618	$ 109,221	$ 69,191	$ 355,761	$ 342,517	$ 1,172,000
Earnings per Share [Abstract]
Basic (in dollars per share)	$ 0.07	$ 0.32	$ 0.32	$ 0.26	$ 0.31	$ 0.16	$ 0.32	$ 0.20	$ 0.98	$ 1.00	$ 3.45
Diluted (in dollars per share)	$ 0.07	$ 0.32	$ 0.31	$ 0.24	$ 0.28	$ 0.14	$ 0.26	$ 0.17	$ 0.95	$ 0.86	$ 2.60