Unaudited Selected Quarterly Financial Data (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																								12 Months Ended
	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, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected quarterly financial data
Insurance premiums written and contract deposits	$ 275,379		$ 306,033		$ 267,703		$ 245,078		$ 279,479		$ 285,206		$ 260,289		$ 242,753		$ 276,113		$ 298,910		$ 259,324		$ 244,102		$ 1,094,193	$ 1,067,727	$ 1,078,449
Total revenues	259,215		251,884		265,637		254,531		255,417		256,548		254,226		244,623		247,645		260,833		245,351		244,473		1,031,267	1,010,814	998,302
Net income	$ 34,287		$ 23,599		$ 25,995		$ 27,012		$ 31,826		$ 32,266		$ 13,103		$ 26,671		$ 32,913		$ 23,637		$ (11,851)		$ 25,807		$ 110,893	$ 103,866	$ 70,506
Basic
Net income per share - basic	$ 0.84		$ 0.59		$ 0.65		$ 0.68		$ 0.81		$ 0.82		$ 0.33		$ 0.67		$ 0.82		$ 0.59		$ (0.30)		$ 0.65		$ 2.75	$ 2.63	$ 1.77
Basic	40,818	[1]	40,001	[1]	39,768	[1]	39,527	[1]	39,339	[1]	39,381	[1]	39,544	[1]	39,794	[1]	39,900	[1]	39,919	[1]	39,893	[1]	39,749	[1]	40,376,562	39,513,540	39,865,815
Diluted
Net income per share - diluted	$ 0.81		$ 0.57		$ 0.63		$ 0.66		$ 0.77		$ 0.78		$ 0.32		$ 0.64		$ 0.79		$ 0.57		$ (0.30)		$ 0.62		$ 2.66	$ 2.51	$ 1.70
Shares of common stock and equivalent shares - weighted average (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]	41,414	[1]	41,451	[1]	39,893	[1]	41,699	[1]	41,633,240	41,388,368	41,436,512

[1] Rounded to thousands.