Quarterly Results of Operations (unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 126,287	$ 117,281	$ 107,130	$ 95,757	$ 71,481	$ 68,353	$ 62,290	$ 74,952	$ 446,455	$ 277,076	$ 540,778
Loss from operations	(8,736)	(10,892)	(12,729)	(18,873)	(34,524)	(29,885)	(26,025)	(23,014)	(51,230)	(113,448)	(166,700)
Income tax benefit (expense)	5,403	(17)	(1,135)	(48)	5,086	1,698	1,990	1,958	4,203	10,732	37,579
Net loss	$ (12,558)	$ (17,227)	$ (20,209)	$ (25,124)	$ (36,081)	$ (34,620)	$ (29,991)	$ (27,699)	$ (75,118)	$ (128,391)	$ (155,140)
Loss per share:
Loss per common share - Basic	$ (0.16)	$ (0.22)	$ (0.26)	$ (0.33)	$ (0.53)	$ (0.53)	$ (0.46)	$ (0.43)	$ (0.97)	$ (1.96)	$ (2.41)
Loss per common share - Diluted	$ (0.16)	$ (0.22)	$ (0.26)	$ (0.33)	$ (0.53)	$ (0.53)	$ (0.46)	$ (0.43)	$ (0.97)	$ (1.96)	$ (2.41)