Selected Quarterly Financial Information (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
Revenues		$ 204,847		$ 188,488		$ 170,581		$ 175,724		$ 174,280		$ 153,408		$ 138,305		$ 136,184		$ 739,640	$ 602,177	$ 854,231
Gross profit (loss)	[1]	37,211		27,181		24,462		32,657		36,782		(26,893)		(73,830)		(49,745)
Net income (loss)		$ 89,155		$ 3,705		$ 9,059		$ 15,929		$ 1,683		$ (24,022)		$ (72,136)		$ (41,149)		$ 117,848	$ (135,624)	$ (1,037,361)
Net income (loss) per common share:
Basic		$ 1.74		$ 0.07		$ 0.18		$ 0.32		$ 0.03	[2]	$ (0.48)	[2]	$ (1.44)	[2]	$ (0.83)	[2]	$ 2.31	$ (2.71)	$ (21.12)
Diluted		$ 1.71	[2]	$ 0.07	[2]	$ 0.17	[2]	$ 0.31	[2]	$ 0.03	[2]	$ (0.48)	[2]	$ (1.44)	[2]	$ (0.83)	[2]	$ 2.28	$ (2.71)	$ (21.12)

[1] Gross income (loss) excludes general and administrative expense, interest expense, (gain) loss on disposition of assets, gain (loss) on derivatives, income taxes, and other income (loss).

[2] The earnings (loss) per share for the year's four quarters does not equal annual income (loss) per share.