Stock-Based Compensation - Non-vested Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted stock unit awards
Restricted Stock Unit (RSU) Awards Outstanding
Aggregate grant date fair value of restricted stock unit awards outstanding	$ 5,515	$ 7,763	$ 6,306
Unrecognized compensation cost related to non-vested restricted stock unit awards	$ 3,879
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 10 months 6 days
Number of Restricted Stock Awards Outstanding
Non-vested restricted stock unit awards outstanding, beginning of year (shares)	3,750,292
Restricted stock unit awards granted (shares)	2,569,039
Restricted stock unit awards forfeited (shares)	(2,050,872)
Restricted stock unit awards vested (shares)	(1,750,084)
Non-vested restricted stock unit awards outstanding, end of year (shares)	2,518,375	3,750,292
Weighted Average Grant Date Fair Value Per Share
Non-vested restricted stock unit awards outstanding, beginning of year (usd per shares)	$ 2.07
Restricted stock unit awards granted (usd per shares)	2.23
Restricted stock unit awards forfeited (usd per shares)	2.08
Restricted stock unit awards vested (usd per shares)	2.11
Non-vested restricted stock unit awards outstanding, end of year (usd per shares)	$ 2.19	$ 2.07
Restricted stock unit awards | Maximum
Restricted Stock Unit (RSU) Awards Outstanding
Vesting period	4 years
Restricted stock units awards outside of equity compensation plan | Chief Executive Officer
Number of Restricted Stock Awards Outstanding
Restricted stock unit awards granted (shares)	186,440