Stock-Based Compensation - Assumptions (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee and non-employee stock options
Stock Based Compensation
Options granted (in shares)			1,512,000
Employee stock options
Valuation of Stock Option Grants to Employees
Expected volatility, minimum	40.77%	62.68%	40.28%	62.68%
Expected volatility, maximum	40.98%	62.93%	40.98%	62.93%
Expected dividend rate	0.00%	0.00%	0.00%	0.00%
Risk free interest rate, minimum	2.70%	1.80%	2.37%	1.80%
Risk free interest rate, maximum	2.86%	1.87%	2.86%	1.91%
Employee stock options | Minimum
Valuation of Stock Option Grants to Employees
Expected term (years)	5 years 7 months 10 days	5 years 1 month 21 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 7 months 13 days	5 years 2 months 12 days	5 years 7 months 13 days	5 years 2 months 12 days
Non-employee stock options
Stock Based Compensation
Options granted (in shares)			110,113
Options unvested (in shares)	8,984		8,984
Valuation of Stock Option Grants to Employees
Expected term (years)		2 years 8 months 9 days
Expected volatility	42.03%	52.18%
Expected volatility, minimum			41.48%	51.69%
Expected volatility, maximum			42.03%	52.18%
Expected dividend rate	0.00%	0.00%	0.00%	0.00%
Risk free interest rate		1.50%
Risk free interest rate, minimum			2.36%	1.42%
Risk free interest rate, maximum	2.58%		2.58%	1.50%
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		2 years 9 months	2 years 8 months 9 days