Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 245,665	$ 250,426	$ 246,964	$ 232,107	$ 237,425	$ 237,083	$ 231,136	$ 229,103	$ 975,162	$ 934,747	$ 925,226
Investment income, net of expenses	38,328	36,380	34,502	32,121	31,276	30,402	29,716	29,477	141,331	120,871	110,666
Realized (losses) gains	(241)	1,114	(1,897)	(329)	458	(50)	(52)	(125)	(1,353)	231	8,921
Other revenue	1,881	2,525	2,431	1,871	2,343	2,925	2,512	2,425	8,708	10,205	17,670
Loss incurred, net	27,685	(1,518)	(13,455)	23,850	(30,996)	29,747	27,339	27,619	36,562	53,709	240,157
Underwriting and other expenses, net	63,239	60,069	57,933	61,895	57,042	56,146	55,292	59,304	243,136	227,784
Loss on debt extinguishment					0	0	65	0	0	65	90,531
Provision for income tax	36,963	49,994	50,708	36,388	218,142	64,440	61,994	84,159	174,053	428,735	172,197
Net income	$ 157,746	$ 181,900	$ 186,814	$ 143,637	$ 27,314	$ 120,027	$ 118,622	$ 89,798	$ 670,097	$ 355,761	$ 342,517
Earnings per Share [Abstract]
Basic (in dollars per share)	$ 0.44	$ 0.50	$ 0.51	$ 0.39	$ 0.07	$ 0.32	$ 0.32	$ 0.26	$ 1.83	$ 0.98	$ 1.00
Diluted (in dollars per share)	$ 0.43	$ 0.49	$ 0.49	$ 0.38	$ 0.07	$ 0.32	$ 0.31	$ 0.24	$ 1.78	$ 0.95	$ 0.86