Net (Loss) Income Per Common Share - Calculation of Diluted Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Numerator for diluted net (loss) income per share:
Net (loss) income	$ (21,324)	$ 22,409	$ (3,535)	$ 77,959	$ 201,700
Less: Series B preferred stock dividends	(4,500)	(4,500)	(9,000)	(9,000)
Less: undistributed earnings attributed to participating securities (diluted calculation)		(9,865)		(37,949)
Diluted net (loss) income attributable to common stockholders	$ (25,824)	$ 8,044	$ (12,535)	$ 31,010
Denominator for diluted net (loss) income per share:
Weighted-average shares outstanding	206,490,690	206,235,839	206,436,048	206,201,937
Weighted-average unvested restricted shares outstanding	(3,857,807)	(3,894,630)	(3,907,045)	(4,002,144)
Effect of dilutive potential shares		34,540,870		35,172,388
Denominator for diluted net (loss) income per share	202,632,883	236,882,079	202,529,003	237,372,181
Diluted net (loss) income per share attributable to common stockholders	$ (0.13)	$ 0.03	$ (0.06)	$ 0.13