Quarterly Financial Data (Unaudited) - Summary of 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, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 725,695	$ 668,008	$ 640,007	$ 625,578	$ 603,662	$ 555,951	$ 525,006	$ 508,912	$ 2,659,288	$ 2,193,531	$ 1,644,275
Insurance costs	576,698	534,481	517,994	483,203	476,779	428,184	400,195	381,157	2,112,376	1,686,315	1,226,585
Operating income	29,609	11,682	5,985	31,041	20,239	21,246	20,029	25,277	78,317	86,791	66,337
Net income	$ 14,095	$ 3,097	$ (1,308)	$ 15,811	$ 7,011	$ 725	$ 6,221	$ 1,540	$ 31,695	$ 15,497	$ 13,147
Basic (in dollars per share)	$ 0.20	$ 0.04	$ (0.02)	$ 0.23	$ 0.10	$ 0.01	$ 0.09	$ 0.03	$ 0.45	$ 0.24	$ 0.26
Diluted (in dollars per share)	$ 0.20	$ 0.04	$ (0.02)	$ 0.22	$ 0.10	$ 0.01	$ 0.09	$ 0.03	$ 0.44	$ 0.22	$ 0.24