Unaudited Selected Quarterly Financial Data (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																								12 Months Ended
	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, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected quarterly financial data
Insurance premiums written and contract deposits	$ 292,241		$ 322,746		$ 292,393		$ 260,275		$ 275,379		$ 306,033		$ 267,703		$ 245,078		$ 279,479		$ 285,206		$ 260,289		$ 242,753
Total revenues	269,157		265,520		264,743		261,265		259,215		251,884		265,637		254,531		255,417		256,548		254,226		244,623		1,060,685	1,031,267	1,010,814
Net income	$ 30,068		$ 25,357		$ 20,452		$ 28,366		$ 34,287		$ 23,599		$ 25,995		$ 27,012		$ 31,826		$ 32,266		$ 13,103		$ 26,671		$ 104,243	$ 110,893	$ 103,866
Basic
Net income	$ 0.72		$ 0.61		$ 0.49		$ 0.69		$ 0.84		$ 0.59		$ 0.65		$ 0.68		$ 0.81		$ 0.82		$ 0.33		$ 0.67		$ 2.50	$ 2.75	$ 2.63
Shares of common stock - weighted average	41,748	[1]	41,514	[1]	41,432	[1]	41,180	[1]	40,818	[1]	40,001	[1]	39,768	[1]	39,527	[1]	39,339	[1]	39,381	[1]	39,544	[1]	39,794	[1]	41,646,281	40,376,562	39,513,540
Diluted
Net income	$ 0.71		$ 0.60		$ 0.48		$ 0.67		$ 0.81		$ 0.57		$ 0.63		$ 0.66		$ 0.77		$ 0.78		$ 0.32		$ 0.64		$ 2.47	$ 2.66	$ 2.51
Shares of common stock and equivalent shares - weighted average (1)	42,362	[1]	42,319	[1]	42,310	[1]	42,259	[1]	42,205	[1]	41,732	[1]	41,395	[1]	41,088	[1]	41,272	[1]	41,138	[1]	41,304	[1]	41,546	[1]	42,230,559	41,633,240	41,388,368

[1] Rounded to thousands.