Net Income Per Common Share - Calculation of Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Numerator for diluted net income per share:
Net income	$ 17,789	$ 55,550	$ 201,700
Less: Series B preferred stock dividends	(4,500)	(4,500)
Less: undistributed earnings attributed to participating securities (diluted calculation)	(8,428)	(27,993)
Diluted net income attributable to common stockholders	$ 4,861	$ 23,057
Denominator for diluted net income per share:
Weighted-average shares outstanding	206,380,800	206,168,035
Weighted-average unvested restricted shares outstanding	(3,956,831)	(4,109,658)
Effect of dilutive potential shares	11,199,990	35,941,341
Denominator for diluted net income per share	213,623,959	237,999,718
Diluted net income per share attributable to common stockholders	$ 0.02	$ 0.10