Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Insurance premiums written and contract deposits	$ 305,186	$ 326,198	$ 319,394	$ 305,735	$ 292,241	$ 322,746	$ 292,393	$ 260,275	$ 275,379	$ 306,033	$ 267,703	$ 245,078
Total revenues	276,106	265,753	268,470	270,119	269,157	265,520	264,743	261,265	259,215	251,884	265,637	254,531	$ 1,080,448	$ 1,060,685	$ 1,031,267
Net income	$ 21,040	$ 21,984	$ 16,183	$ 34,275	$ 30,068	$ 25,357	$ 20,452	$ 28,366	$ 34,287	$ 23,599	$ 25,995	$ 27,012	$ 93,482	$ 104,243	$ 110,893
Basic
Net income	$ 0.51	$ 0.53	$ 0.39	$ 0.82	$ 0.72	$ 0.61	$ 0.49	$ 0.69	$ 0.84	$ 0.59	$ 0.65	$ 0.68	$ 2.23	$ 2.5	$ 2.75
Shares of common stock - weighted average (1)	41,564,000	41,852,000	41,990,000	41,950,000	41,748,000	41,514,000	41,432,000	41,180,000	40,818,000	40,001,000	39,768,000	39,527,000	41,914,864	41,646,281	40,376,562
Diluted
Net income	$ 0.50	$ 0.52	$ 0.38	$ 0.81	$ 0.71	$ 0.60	$ 0.48	$ 0.67	$ 0.81	$ 0.57	$ 0.63	$ 0.66	$ 2.20	$ 2.47	$ 2.66
Shares of common stock and equivalent shares - weighted average (1)	42,127,000	42,305,000	42,425,000	42,300,000	42,362,000	42,319,000	42,310,000	42,259,000	42,205,000	41,732,000	41,395,000	41,088,000	42,424,806	42,230,559	41,633,240