Net Income (Loss) Per Common Share - Calculation of Diluted 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 diluted 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 (diluted calculation)		(8,428)
Diluted net (loss) income attributable to common stockholders	$ (5,783)	$ 4,861
Denominator for diluted 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)
Effect of dilutive potential shares		11,199,990
Denominator for diluted net (loss) income per share	202,692,144	213,623,959
Diluted net (loss) income per share attributable to common stockholders	$ (0.03)	$ 0.02