Stock-Based Compensation - Assumptions (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Employee and non-employee stock options
Stock Based Compensation
Options granted (in shares)	1,195,000
Employee stock options
Valuation of Stock Option Grants to Employees
Expected volatility, minimum	42.53%	40.28%
Expected volatility, maximum	42.75%	40.88%
Expected dividend rate	0.00%	0.00%
Risk free interest rate, minimum	2.26%	2.37%
Risk free interest rate, maximum	2.60%	2.70%
Employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term (years)	5 years 3 months 29 days	5 years 2 months 27 days
Employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term (years)	5 years 6 months 11 days	5 years 7 months 2 days
Non-employee stock options
Stock Based Compensation
Options granted (in shares)	122,606
Options unvested (in shares)	28,793
Valuation of Stock Option Grants to Employees
Expected volatility, minimum		41.48%
Expected volatility, maximum		41.71%
Expected dividend rate		0.00%
Risk free interest rate, minimum		2.36%
Risk free interest rate, maximum		2.40%
Non-employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term (years)		2 years 8 months 19 days
Non-employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term (years)		2 years 8 months 27 days