Supplemental Financial Information Quarterly Results (unaudited) - Schedule of Unaudited Quarterly Results (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
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 66,900	$ 48,100	$ 67,300	$ 56,900	$ 101,600	$ 76,600	$ 90,500	$ 66,900	$ 239,182	$ 335,519	$ 261,560
Total operating expenses	63,900	57,500	56,900	54,000	68,400	56,500	59,800	59,700	232,282	244,411	215,787
Total operating income	3,000	(9,400)	10,400	2,900	33,200	20,100	30,700	7,200	6,900	91,108	45,773
Interest expense	4,700	900	800	800	800	900	800	700	7,198	3,227	2,478
Income (loss) before taxes	(1,700)	(10,300)	9,600	2,100	32,400	19,200	29,900	6,500	(298)	87,881	43,295
Provision (benefit) for taxes	24,500	(4,400)	3,300	2,900	8,700	6,100	10,200	2,100	26,353	27,119	17,697
Net income allocated to common stockholders	$ (26,200)	$ (5,900)	$ 6,300	$ (800)	$ 23,700	$ 13,100	$ 19,700	$ 4,400	$ (26,651)	$ 60,762	$ 25,598
Earnings (loss) per share:
Basic (usd per share)	$ (0.85)	$ (0.18)	$ 0.20	$ (0.02)	$ 0.74	$ 0.41	$ 0.62	$ 0.14	$ (0.83)	$ 1.90	$ 0.82
Diluted (usd per share)	(0.85)	(0.18)	0.20	(0.02)	0.74	0.41	0.62	0.14	(0.83)	1.89	0.82
Dividends declared per share (usd per share)	$ 0.05	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.40	$ 1.8	$ 1.80