Stock-Based Compensation - Restricted Stock Unit Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Vested (in shares)	(981,633)	(381,916)
Restricted Stock
Shares
Balance at beginning of year (in shares)	1,463,796	1,204,883	977,069
Granted (in shares)	1,090,000	845,862	789,500
Forfeited (in shares)	(213,001)	(205,033)	(209,748)
Vested (in shares)	(981,633)	(381,916)	(351,938)
Balance at end of year (in shares)	1,359,162	1,463,796	1,204,883
Weighted Average Grant Date Fair Value
Balance at beginning of year (in dollars per share)	$ 2.82	$ 4.21	$ 5.36
Granted (in dollars per share)	0.97	1.88	3.33
Forfeited (in dollars per share)	2.99	4.25	4.62
Vested (in dollars per share)	2.85	4.36	5.17
Balance at end of year (in dollars per share)	$ 1.28	$ 2.82	$ 4.21