Stock-Based Compensation - Summary of Stock Option Activity (Details) - 2011 Equity Incentive Plan - Employee Stock Option - $ / shares shares in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options outstanding, number of shares (in shares)	7,120
Granted, number of shares (in shares)	2,103
Forfeited or expired, number of shares (in shares)	(636)
Options outstanding, number of shares (in shares)	8,587	7,120
Options exercisable, number of shares (in shares)	3,357
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.83
Granted, weighted average exercise price (in dollars per share)	0.78
Forfeited or expired, weighted average exercise price (in dollars per share)	3.99
Options outstanding, weighted average exercise price (in dollars per share)	3.07	$ 3.83
Options exercisable, weighted average exercise price (in dollars per share)	$ 5.26
Options outstanding, weighted average remaining contractual life (Year)	7 years 9 months 7 days	6 years 9 months 21 days
Options exercisable, weighted average remaining contractual life (Year)	5 years 9 months 3 days