Stock-Based Compensation - Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock option activity
Options outstanding- Beginning	24,218,000	21,458,000
Options granted	469,000	9,294,000
Options exercised	(201,447)	(87,591)	(1,438,671)
Options canceled/forfeited	(8,711,000)	(6,446,000)
Options outstanding- Ending	15,775,000	24,218,000	21,458,000
Weighted Average Exercise Price Per Share
Options outstanding- Beginning (in dollars per share)	$ 2.16	$ 1.61
Options granted (in dollars per share)	0.54	2.54
Options exercised (in dollars per share)	0.53	1.26
Options canceled/forfeited (in dollars per share)	$ 1.95	2.48
Options outstanding- Ending (in dollars per share)		$ 2.16	$ 1.61
Option awards - additional information
Options vested and expected to vest	15,775,000
Options vested and expected to vest (in dollars per share)	$ 2.27
Number of options exercisable	12,222,000
Weighted-average exercise price of exercisable options	$ 2.19
Weighted-average remaining contractual life of outstanding stock options	6 years 6 months 3 days	6 years 7 months 20 days
Weighted-average remaining contractual life of exercisable options	6 years 1 month 6 days