Net Income (Loss) Per Common Share - Calculation of Basic Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Numerator for basic net (loss) income per share:
Net (loss) income	$ (5,583)	$ 17,789	$ 25,882
Less: Series B preferred stock dividends	(200)	(4,500)
Less: undistributed earnings attributed to participating securities (basic calculation)		(8,445)
Basic net (loss) income attributable to common stockholders	$ (5,783)	$ 4,844
Denominator for basic net (loss) income per share:
Weighted-average shares outstanding	206,714,132	206,380,800
Weighted-average unvested restricted shares outstanding	(4,021,988)	(3,956,831)
Denominator for basic net (loss) income per share	202,692,144	202,423,969
Basic net (loss) income per share attributable to common stockholders	$ (0.03)	$ 0.02