Stock-Based Compensation - Assumptions (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee and non-employee stock options
Stock Based Compensation
Options granted (in shares)			1,697,000
Employee stock options
Valuation of Stock Option Grants to Employees
Expected volatility, minimum	42.71%	40.77%	42.53%	40.28%
Expected volatility, maximum	43.49%	40.98%	43.49%	40.98%
Expected dividend rate	0.00%	0.00%	0.00%	0.00%
Risk free interest rate, minimum	1.78%	2.70%	1.78%	2.37%
Risk free interest rate, maximum	2.51%	2.86%	2.60%	2.86%
Employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term (years)	5 years 3 months 18 days	5 years 7 months 10 days	5 years 3 months 18 days	5 years 2 months 27 days
Employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term (years)	10 years	5 years 7 months 13 days	10 years	5 years 7 months 13 days
Non-employee stock options
Stock Based Compensation
Options granted (in shares)			105,584
Options unvested (in shares)	24,721		24,721
Valuation of Stock Option Grants to Employees
Expected term (years)		2 years 9 months
Expected volatility		42.03%
Expected volatility, minimum				41.48%
Expected volatility, maximum				42.03%
Expected dividend rate		0.00%		0.00%
Risk free interest rate		2.58%
Risk free interest rate, minimum				2.36%
Risk free interest rate, maximum				2.58%
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 9 months