Stock-based Compensation - Option Valuation Method (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Weighted average expected volatility, employees	52.31%	51.38%
Weighted average expected volatility, non-employees directors	58.65%	53.86%
Weighted average risk-free interest rate, employees	2.64%	2.07%
Weighted average risk-free interest rate, non-employees directors	2.44%	2.01%
Weighted average expected term (in years), employees	6 years 7 days	6 years 14 days
Weighted average expected term (in years), non-employees directors	5 years 3 months 10 days	5 years 4 months 13 days