Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 266,267	$ 267,857	$ 247,102	$ 249,762	$ 245,665	$ 250,426	$ 246,964	$ 232,107	$ 1,030,988	$ 975,162	$ 934,747
Investment income, net of expenses	41,322	42,715	42,423	40,585	38,328	36,380	34,502	32,121	167,045	141,331	120,871
Realized (losses) gains	1,320	4,205	307	(526)	(241)	1,114	(1,897)	(329)	5,306	(1,353)	231
Other revenue	2,717	3,606	2,485	1,830	1,881	2,525	2,431	1,871	10,638	8,708	10,205
Loss incurred, net	23,690	33,985	21,836	39,064	27,685	(1,518)	(13,455)	23,850	118,575	36,562	53,709
Underwriting and other expenses, net	65,227	61,278	59,270	61,650	63,239	60,069	57,933	61,895	247,425	243,136
Loss on debt extinguishment									0	0	65
Provision for income tax	45,599	46,186	43,433	38,996	36,963	49,994	50,708	36,388	174,214	174,053	428,735
Net income	$ 177,110	$ 176,934	$ 167,778	$ 151,941	$ 157,746	$ 181,900	$ 186,814	$ 143,637	$ 673,763	$ 670,097	$ 355,761
Earnings per Share [Abstract]
Basic (in dollars per share)	$ 0.51	$ 0.50	$ 0.47	$ 0.43	$ 0.44	$ 0.50	$ 0.51	$ 0.39	$ 1.91	$ 1.83	$ 0.98
Diluted (in dollars per share)	$ 0.49	$ 0.49	$ 0.46	$ 0.42	$ 0.43	$ 0.49	$ 0.49	$ 0.38	$ 1.85	$ 1.78	$ 0.95