Employee Benefit Plans - Stock Activity, Weighted Average Exercise Price (Details) - Stock options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares
Options outstanding, beginning balance (in shares)	21,957,150
Granted (in shares)	3,221,892	2,679,667	5,485,863
Exercised (in shares)	(1,151,307)
Forfeited (in shares)	(934,195)
Expired (in shares)	(79,950)
Options outstanding, end balance (in shares)	23,013,590	21,957,150
Options exercisable (in shares)	16,608,362
Options unvested (in shares)	6,405,228
Weighted Average Exercise Price
Options outstanding, beginning balance, weighted-average exercise price	$ 2.51
Granted, weighted-average exercise price	2.40
Exercised, weighted-average exercise price	1.56
Forfeited, weighted-average exercise price	3.50
Expired, weighted-average exercise price	9.43
Options outstanding, end balance, weighted-average exercise price	2.48	$ 2.51
Options exercisable, weighted-average exercise price	2.57
Options unvested, weighted-average exercise price	$ 2.22
Stock option activity additional disclosures
Options outstanding, weighted-average remaining contractual term	6 years 7 months 6 days	7 years 1 month 6 days
Options exercisable, weighted-average remaining contractual term	5 years 9 months 18 days
Options unvested, weighted-average remaining contractual term	8 years 10 months 24 days
Options outstanding, aggregate intrinsic value	$ 22,277	$ 1,432
Options exercisable, aggregate intrinsic value	16,285
Options unvested, aggregate intrinsic value	$ 5,992
Weighted-average grant date fair value of options granted (per share)	$ 1.67	$ 1.55	$ 1.67
Unrecognized compensation cost	$ 8,100
Period for recognition	3 years
Fair value of stock options that vested during the period	$ 6,100	$ 7,100