Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ 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, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 811,071	$ 770,457	$ 745,846	$ 732,939	$ 725,695	$ 668,008	$ 640,007	$ 625,578	$ 3,060,313	$ 2,659,288	$ 2,193,531
Insurance costs	637,968	609,422	596,673	569,689	576,698	534,481	517,994	483,203	2,413,752	2,112,376	1,686,315
Operating income	42,717	28,972	26,367	25,902	29,609	11,682	5,985	31,041	123,958	78,317	86,791
Net income	$ 22,966	$ 14,581	$ 12,282	$ 11,577	$ 14,095	$ 3,097	$ (1,308)	$ 15,811	$ 61,406	$ 31,695	$ 15,497
Basic net income (loss) per share (in dollars per share)	$ 0.34	$ 0.21	$ 0.17	$ 0.16	$ 0.20	$ 0.04	$ (0.02)	$ 0.23	$ 0.88	$ 0.45	$ 0.24
Diluted net income (loss) per share (in dollars per share)	$ 0.32	$ 0.20	$ 0.17	$ 0.16	$ 0.20	$ 0.04	$ (0.02)	$ 0.22	$ 0.85	$ 0.44	$ 0.22