Equity Plans - Non-employees Stock Options Valuation Assumptions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Plans
Expected volatility		52.48%
Stock-based compensation expense	$ 4,241,000	$ 2,130,000	$ 99,000
Non-employee stock options
Equity Plans
Expected term			6 years 9 months 18 days
Expected volatility			59.80%
Expected volatility, Minimum	61.70%	62.50%
Expected volatility, Maximum	65.40%	62.80%
Risk-free interest rate			1.95%
Risk-free interest rate, Minimum	2.17%	1.29%
Risk-free interest rate, Maximum	2.33%	1.79%
Options granted	0	59,647	4,816
Stock-based compensation expense	$ 263,000	$ 505,000	$ 15,000
Non-employee stock options | Minimum
Equity Plans
Expected term	6 years 11 months 19 days	6 years 7 months 2 days
Non-employee stock options | Maximum
Equity Plans
Expected term	7 years 4 months 24 days	9 years 11 months 19 days