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Stock-Based Compensation - Schedule of Outstanding and Exercisable Options (Details)
3 Months Ended
Mar. 31, 2019
$ / shares
shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Stock options outstanding (in shares) | shares 6,105,619
Weighted average exercise price (in dollars per share) $ 3.68
Weighted average remaining contractual life 6 years 11 months 4 days
Stock options exercisable (in shares) | shares 3,601,093
Weighted average exercise price (in dollars per share) $ 4.65
$0.79 - $1.50  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Range of exercise prices (in dollars per share) 0.79
Range of exercise prices (in dollars per share) $ 1.50
Stock options outstanding (in shares) | shares 1,652,250
Weighted average exercise price (in dollars per share) $ 1.08
Weighted average remaining contractual life 3 years 5 months 19 days
Stock options exercisable (in shares) | shares 1,510,000
Weighted average exercise price (in dollars per share) $ 1.06
$1.51 - $5.50  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Range of exercise prices (in dollars per share) 1.51
Range of exercise prices (in dollars per share) $ 5.50
Stock options outstanding (in shares) | shares 2,642,313
Weighted average exercise price (in dollars per share) $ 2.17
Weighted average remaining contractual life 9 years 2 months 19 days
Stock options exercisable (in shares) | shares 452,961
Weighted average exercise price (in dollars per share) $ 3.17
$5.51 - $10.67  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Range of exercise prices (in dollars per share) 5.51
Range of exercise prices (in dollars per share) $ 10.67
Stock options outstanding (in shares) | shares 1,811,056
Weighted average exercise price (in dollars per share) $ 8.25
Weighted average remaining contractual life 6 years 8 months 23 days
Stock options exercisable (in shares) | shares 1,638,132
Weighted average exercise price (in dollars per share) $ 8.36