Financial Results by Quarter (Unaudited) Financial Results by Quarter (Unaudited) (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 Data [Abstract]
Revenues	$ 155,681		$ 174,027		$ 141,495		$ 127,896		$ 137,989		$ 174,475		$ 128,747		$ 134,125		$ 599,099	$ 575,336	$ 606,950
Operating income (loss)	25,716		38,994		(34,228)		5,072		9,855		27,714		7,311		14,027		35,554	58,907	132,506
Net income (loss)	$ 14,373		$ 27,646		$ (31,238)		$ 157		$ 3,549		$ 21,798		$ 711		$ 5,656		$ 10,938	$ 31,714	$ 89,910
Basic (in dollars per share)	$ 0.35	[1]	$ 0.68	[1]	$ (0.77)	[1]	$ 0	[1]	$ 0.09	[1]	$ 0.53	[1]	$ 0.02	[1]	$ 0.14	[1]	$ 0.27	$ 0.78	$ 2.21
Diluted (in dollars per share)	$ 0.35	[1]	$ 0.68	[1]	$ (0.77)	[1]	$ 0	[1]	$ 0.09	[1]	$ 0.53	[1]	$ 0.02	[1]	$ 0.14	[1]	$ 0.27	$ 0.77	$ 2.20

[1] Per share amounts are computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the total computed for the year.