Net Income (Loss) Per Common Share - Calculation of Diluted Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Numerator for diluted net income (loss) per share:
Net income (loss)	$ 38,411	$ (16,645)	$ 34,876	$ 61,314	$ 201,700
Less: Series B preferred stock dividends	(4,500)	(4,500)	(13,500)	(13,500)
Less: undistributed earnings attributed to participating securities (diluted calculation)	(21,541)		(13,581)	(26,241)
Diluted net income (loss) attributable to common stockholders	$ 12,370	$ (21,145)	$ 7,795	$ 21,573
Denominator for diluted net income (loss) per share:
Weighted-average shares outstanding	206,714,132	206,380,800	206,529,762	206,261,993
Weighted-average unvested restricted shares outstanding	(4,053,640)	(4,039,591)	(3,956,447)	(4,014,718)
Effect of dilutive potential shares	10,065,629		10,065,629	35,327,739
Denominator for diluted net income (loss) per share	212,726,121	202,341,209	212,638,944	237,575,014
Diluted net income (loss) per share attributable to common stockholders	$ 0.06	$ (0.10)	$ 0.04	$ 0.09