Stock-Based Compensation - Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee and non-employee stock options
Stock based compensation expense
Options granted (in shares)	409,000
Stock-based compensation expense	$ 7,417	$ 4,051
Unrecognized compensation expense	$ 96,700
Unrecognized compensation expense, weighted average period of recognition	3 years 1 month 6 days
Employee and non-employee stock options | Cost of revenues
Stock based compensation expense
Stock-based compensation expense	$ 314	168
Employee and non-employee stock options | Research and development
Stock based compensation expense
Stock-based compensation expense	1,842	894
Employee and non-employee stock options | Selling, general and administrative
Stock based compensation expense
Stock-based compensation expense	5,261	2,989
Employee stock options
Stock based compensation expense
Stock-based compensation expense	7,248	3,708
Employee stock options | Cost of revenues
Stock based compensation expense
Stock-based compensation expense	314	168
Employee stock options | Research and development
Stock based compensation expense
Stock-based compensation expense	1,682	894
Employee stock options | Selling, general and administrative
Stock based compensation expense
Stock-based compensation expense	$ 5,252	2,646
Non-employee stock options
Stock based compensation expense
Options granted (in shares)	72,849
Options unvested (in shares)	23,126
Stock-based compensation expense	$ 169	343
Non-employee stock options | Research and development
Stock based compensation expense
Stock-based compensation expense	160
Non-employee stock options | Selling, general and administrative
Stock based compensation expense
Stock-based compensation expense	$ 9	$ 343