Equity Plans - Employee Stock Options Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Plans
Expected volatility		52.48%
Employee stock options
Equity Plans
Expected term			5 years 10 months 21 days
Expected volatility			59.80%
Expected volatility, Minimum	61.60%	62.50%
Expected volatility, Maximum	65.40%	64.80%
Risk-free interest rate, Minimum	1.88%	1.27%	1.57%
Risk-free interest rate, Maximum	2.24%	1.79%	1.58%
Employee stock options | Minimum
Equity Plans
Expected term	5 years 6 months	4 years 1 month 28 days
Employee stock options | Maximum
Equity Plans
Expected term	6 years 29 days	5 years 11 months 12 days