Net Income (Loss) Per Common Share - Calculation of Basic Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Numerator for basic net loss per share:
Net loss	$ (1,996)	$ (21,324)	$ (7,579)	$ (3,535)	$ 25,882
Less: Series B preferred stock dividends		(4,500)	(200)	(9,000)
Basic net loss attributable to common stockholders	$ (1,996)	$ (25,824)	$ (7,779)	$ (12,535)
Denominator for basic net loss per share:
Weighted-average shares outstanding	209,902,759	206,490,690	208,317,254	206,436,048
Weighted-average unvested restricted shares outstanding	(3,841,073)	(3,857,807)	(3,931,031)	(3,907,045)
Denominator for basic net loss per share	206,061,686	202,632,883	204,386,223	202,529,003
Basic net loss per share attributable to common stockholders	$ (0.01)	$ (0.13)	$ (0.04)	$ (0.06)