Stock-Based Compensation - Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee and non-employee stock options
Stock based compensation expense
Options granted (in shares)	457,000
Stock-based compensation expense	$ 50,171	$ 28,624	$ 14,198
Unrecognized compensation expense	$ 118,400
Unrecognized compensation expense, weighted average period of recognition	2 years 10 months 24 days
Employee and non-employee stock options | Cost of revenues
Stock based compensation expense
Stock-based compensation expense	$ 1,691	937	569
Employee and non-employee stock options | Research and development
Stock based compensation expense
Stock-based compensation expense	11,424	5,354	4,043
Employee and non-employee stock options | Selling, general and administrative
Stock based compensation expense
Stock-based compensation expense	37,056	22,333	9,586
Employee stock options
Stock based compensation expense
Stock-based compensation expense	49,215	27,989	14,081
Employee stock options | Cost of revenues
Stock based compensation expense
Stock-based compensation expense	1,691	905	564
Employee stock options | Research and development
Stock based compensation expense
Stock-based compensation expense	10,777	5,354	4,043
Employee stock options | Selling, general and administrative
Stock based compensation expense
Stock-based compensation expense	$ 36,747	21,730	9,474
Non-employee stock options
Stock based compensation expense
Options granted (in shares)	45,577
Options unvested (in shares)	5,366
Stock-based compensation expense	$ 956	635	117
Non-employee stock options | Cost of revenues
Stock based compensation expense
Stock-based compensation expense		32	5
Non-employee stock options | Research and development
Stock based compensation expense
Stock-based compensation expense	647
Non-employee stock options | Selling, general and administrative
Stock based compensation expense
Stock-based compensation expense	$ 309	$ 603	$ 112