Net Income (Loss) Per Common Share - Calculation of Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for diluted net income (loss) per share:
Net income (loss)													$ 25,882	$ 201,700	$ (61,833)
Less: Series B preferred stock dividends													(18,050)	(18,000)	(17,748)
Less: undistributed earnings attributed to participating securities (diluted calculation)													(5,417)	(112,115)
Diluted net income (loss) attributable to common stockholders													$ 2,415	$ 71,585	$ (79,581)
Denominator for diluted net income (loss) per share:
Weighted-average shares outstanding													206,576,233	206,291,857	204,776,405
Weighted-average unvested restricted shares outstanding													(3,980,945)	(4,020,970)	(3,029,792)
Effect of dilutive potential shares													10,065,629	33,135,473
Denominator for diluted net income (loss) per share													212,660,917	235,406,360	201,746,613
Diluted net income (loss) per share attributable to common stockholders	$ (0.07)	$ 0.06	$ (0.13)	$ 0.02	$ 0.23	$ (0.10)	$ 0.03	$ 0.10	$ 0.01	$ (0.04)	$ (0.39)	$ 0.00	$ 0.01	$ 0.30	$ (0.39)