Unaudited Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																								12 Months Ended
	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, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance premiums written and contract deposits	$ 315,917		$ 351,534		$ 311,879		$ 283,169		$ 305,186		$ 326,198		$ 319,394		$ 305,735		$ 292,241		$ 322,746		$ 292,393		$ 260,275
Total revenues	282,873		291,176		283,558		271,303		276,106		265,753		268,470		270,119		269,157		265,520		264,743		261,265		$ 1,128,910	$ 1,080,448	$ 1,060,685
Net income	$ 19,823		$ 26,923		$ 11,866		$ 25,153		$ 21,040		$ 21,984		$ 16,183		$ 34,275		$ 30,068		$ 25,357		$ 20,452		$ 28,366		$ 83,765	$ 93,482	$ 104,243
Basic
Net income	$ 0.48		$ 0.66		$ 0.29		$ 0.61		$ 0.51		$ 0.53		$ 0.39		$ 0.82		$ 0.72		$ 0.61		$ 0.49		$ 0.69		$ 2.04	$ 2.23	$ 2.5
Weighted average number of common shares during the period (in thousands)	41,093	[1]	41,092	[1]	41,082	[1]	41,297	[1]	41,564	[1]	41,852	[1]	41,990	[1]	41,950	[1]	41,748	[1]	41,514	[1]	41,432	[1]	41,180	[1]	41,158,349	41,914,864	41,646,281
Diluted
Net income	$ 0.48		$ 0.65		$ 0.29		$ 0.61		$ 0.50		$ 0.52		$ 0.38		$ 0.81		$ 0.71		$ 0.60		$ 0.48		$ 0.67		$ 2.02	$ 2.2	$ 2.47
Diluted	41,482	[1]	41,347	[1]	41,314	[1]	41,492	[1]	42,127	[1]	42,305	[1]	42,425	[1]	42,300	[1]	42,362	[1]	42,319	[1]	42,310	[1]	42,259	[1]	41,475,516	42,424,806	42,230,559

[1] Rounded to thousands.