Employee Benefit Plans - Stock Activity, Weighted Average Exercise Price (Details) - Stock options - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Shares      
Options outstanding, beginning balance (in shares) 19,872,029    
Granted (in shares) 2,679,667 5,485,863 3,702,500
Exercised (in shares) (276,668)    
Forfeited (in shares) (246,378)    
Expired (in shares) (71,500)    
Options outstanding, end balance (in shares) 21,957,150 19,872,029  
Options exercisable (in shares) 14,964,985    
Options unvested (in shares) 6,992,165    
Weighted Average Exercise Price      
Options outstanding, beginning balance, weighted-average exercise price $ 2.66    
Granted, weighted-average exercise price 1.96    
Exercised, weighted-average exercise price 0.63    
Forfeited, weighted-average exercise price 2.91    
Expired, weighted-average exercise price 29.51    
Options outstanding, end balance, weighted-average exercise price 2.51 $ 2.66  
Options exercisable, weighted-average exercise price 2.74    
Options unvested, weighted-average exercise price $ 2.01    
Stock option activity additional disclosures      
Options outstanding, weighted-average remaining contractual term 7 years 1 month 6 days 7 years 8 months 12 days  
Options exercisable, weighted-average remaining contractual term 6 years 3 months 18 days    
Options unvested, weighted-average remaining contractual term 8 years 10 months 24 days    
Options outstanding, aggregate intrinsic value $ 1,432    
Options exercisable, aggregate intrinsic value 1,431    
Options unvested, aggregate intrinsic value $ 1,467    
Weighted-average grant date fair value of options granted (per share) $ 1.55 $ 1.67 $ 1.39
Unrecognized compensation cost $ 8,800    
Period for recognition 3 years    
Fair value of stock options that vested during the period $ 7,100 $ 9,600