Net Income (Loss) Per Share - Computation of Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income (loss)	$ (1,470)	$ 7,104	$ (4,444)	$ 5,846
Less: preferred stock dividends	3	3	9	9
Net income (loss) attributable to common stockholders	$ (1,473)	$ 7,101	$ (4,453)	$ 5,837
Denominator:
Weighted-average number of shares of common stock for basic net income (loss) per share (in shares)	48,851,000	36,897,000	47,489,000	37,078,000
Add effect of dilutive securities:
Unvested restricted stock (in shares)			0
Shares of common stock issuable upon conversion of preferred stock (in shares)	0	79,000	0	79,000
Weighted-average shares outstanding - diluted (in shares)	48,851,000	37,897,000	47,489,000	38,078,000
Basic net income (loss) per share (in dollars per share)	$ (0.03)	$ 0.19	$ (0.09)	$ 0.16
Diluted net income (loss) per share (in dollars per share)	$ (0.03)	$ 0.19	$ (0.09)	$ 0.15
Restricted Stock Units (RSUs)
Add effect of dilutive securities:
Unvested restricted stock (in shares)	0	672,000	0	672,000
Restricted stock
Add effect of dilutive securities:
Unvested restricted stock (in shares)	0	249,000	0	249,000