Stock-based Compensation - Options (Details) - Options - 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
Number of options outstanding [Roll Forward]
Options outstanding, beginning of period (in shares)			2,666,000
Granted (in shares)	0	0	0	60,000
Exercised (in shares)			(66,000)
Canceled and expired (in shares)			(2,000)
Options outstanding, end of period (in shares)	2,598,000		2,598,000		2,666,000
Options vested and unvested options expected to vest, net of forfeitures, end of period (in shares)	2,576,000		2,576,000
Options exercisable, end of period (in shares)	1,771,000		1,771,000
Weighted average exercise price [Roll Forward]
Options outstanding, beginning of period (in dollars per share)			$ 3.81
Exercised (in dollars per share)			2.68
Canceled and expired (in dollars per share)			5.21
Options outstanding, end of period (in dollars per share)	$ 3.83		3.83		$ 3.81
Options vested and unvested options expected to vest, net of forfeitures (in dollars per share)	3.81		3.81
Options exercisable, end of period (in dollars per share)	$ 3.42		$ 3.42
Weighted-average grant date fair value of stock options granted (in dollars per share)				$ 2.76
Weighted average Remaining Contractual Life [Abstract]
Options outstanding			6 years 5 months 9 days		6 years 10 months 13 days
Options vested and unvested options expected to vest, net of forfeitures, end of period			6 years 5 months 1 day
Option exercisable, end of period			5 years 8 months 16 days
Aggregate Intrinsic Value [Abstract]
Options outstanding, beginning of period			$ 13,149
Options outstanding, end of period	$ 8,715		8,715		$ 13,149
Options vested and expected to vest, end of period	8,678		8,678
Options exercisable, end of period	$ 6,452		$ 6,452
Closing price (in dollars per share)	$ 7.05		$ 7.05