Stock-Based Compensation - Assumptions (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
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	40.59%	62.27%	40.28%	62.27%
Expected volatility, maximum	41.05%	62.49%	41.05%	62.93%
Expected dividend rate	0.00%	0.00%	0.00%	0.00%
Risk free interest rate, minimum	2.75%	1.67%	2.37%	1.67%
Risk free interest rate, maximum	2.97%	1.90%	2.97%	1.90%
Employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term (years)	5 years 4 months 6 days	5 years 2 months 19 days	5 years 2 months 27 days	5 years 1 month 21 days
Employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term (years)	5 years 6 months	5 years 2 months 23 days	5 years 7 months 13 days	5 years 2 months 23 days
Non-employee stock options
Stock Based Compensation
Options granted (in shares)			141,784
Options unvested (in shares)	47,541		47,541
Valuation of Stock Option Grants to Employees
Expected term (years)	2 years 9 months 4 days	2 years 8 months 12 days
Expected volatility	41.36%	51.28%
Expected volatility, minimum			41.36%	51.28%
Expected volatility, maximum			42.03%	52.18%
Expected dividend rate	0.00%	0.00%	0.00%	0.00%
Risk free interest rate		1.58%
Risk free interest rate, minimum			2.36%	1.42%
Risk free interest rate, maximum	2.86%		2.86%	1.58%
Non-employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term (years)			2 years 8 months 19 days	2 years 8 months 1 day
Non-employee stock options | Maximum
Valuation of Stock Option Grants to Employees
Expected term (years)			2 years 9 months 4 days	2 years 8 months 12 days