Net Income (Loss) Per Common Share - Calculation of Basic 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 basic 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 (basic calculation)	(21,541)		(13,581)	(27,625)
Basic net income (loss) attributable to common stockholders	$ 12,370	$ (21,145)	$ 7,795	$ 20,189
Denominator for basic 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)
Denominator for basic net income (loss) per share	202,660,492	202,341,209	202,573,315	202,247,275
Basic net income (loss) per share attributable to common stockholders	$ 0.06	$ (0.10)	$ 0.04	$ 0.10