Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unaudited Quarterly Information
Total revenues	$ 280,772	$ 205,739	$ 201,194	$ 155,548	$ 261,692	$ 205,427	$ 188,096	$ 124,090	$ 843,253	$ 779,305	$ 517,766
Total expenses	265,728	208,088	199,725	151,380	244,196	193,492	180,800	123,231	824,921	741,719	505,380
Income before income taxes	15,044	(2,349)	1,469	4,168	17,496	11,935	7,296	859	18,332	37,586	12,386
Less: income tax expense (benefit)	38,589	(872)	822	1,729	438	38	(110,065)	68	(40,268)	109,521	(436)
Net income (loss)	$ (23,545)	$ (1,477)	$ 647	$ 2,439	$ 17,058	$ 11,897	$ 117,361	$ 791	$ (21,936)	$ 147,107	$ 11,950
Earnings (loss) per share:
Basic earnings (loss) per share (in dollars per share)	$ (1.05)	$ (0.07)	$ 0.03	$ 0.11	$ 0.77	$ 0.53	$ 5.24	$ 0.04	$ (0.98)	$ 6.58	$ 0.54
Diluted earnings (loss) per share (in dollars per share)	$ (1.05)	$ (0.07)	$ 0.03	$ 0.11	$ 0.68	$ 0.49	$ 4.45	$ 0.04	$ (0.98)	$ 5.66	$ 0.54