Unaudited Selected 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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Insurance premiums written and contract deposits	$ 300,416	$ 318,355	$ 311,614	$ 296,732	$ 315,917	$ 351,534	$ 311,879	$ 283,169	$ 305,186	$ 326,198	$ 319,394	$ 305,735
Total revenues	302,993	289,817	291,436	287,304	282,873	291,176	283,558	271,303	276,106	265,753	268,470	270,119	$ 1,171,550	$ 1,128,910	$ 1,080,448
Net income	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 19,823	$ 26,923	$ 11,866	$ 25,153	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 169,459	$ 83,765	$ 93,482
Basic
Net income - basic (in usd per share)	$ 3.03	$ 0.64	$ 0.05	$ 0.37	$ 0.48	$ 0.66	$ 0.29	$ 0.61	$ 0.51	$ 0.53	$ 0.39	$ 0.82	$ 4.10	$ 2.04	$ 2.23
Shares of common stock - weighted average basic (in shares)	41,419,000,000	41,433,000,000	41,368,000,000	41,135,000,000	41,093,000,000	41,092,000,000	41,082,000,000	41,297,000,000	41,564,000,000	41,852,000,000	41,990,000,000	41,950,000,000	41,364,546	41,158,349	41,914,864
Diluted
Net income - diluted (in usd per share)	$ 3.00	$ 0.64	$ 0.05	$ 0.37	$ 0.48	$ 0.65	$ 0.29	$ 0.61	$ 0.50	$ 0.52	$ 0.38	$ 0.81	$ 4.08	$ 2.02	$ 2.20
Shares of common stock and equivalent shares - weighted average diluted (in shares)	41,718,000,000	41,575,000,000	41,493,000,000	41,342,000,000	41,482,000,000	41,347,000,000	41,314,000,000	41,492,000,000	42,127,000,000	42,305,000,000	42,425,000,000	42,300,000,000	41,564,979	41,475,516	42,424,806