Income Taxes - Provision Difference from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective tax rate of provision for income taxes difference from federal statutory rate
Federal statutory income tax rate (as a percent)	21.00%	21.00%	34.00%
State taxes, net of federal benefit (as a percent)	1.20%	7.00%	0.50%
Research and development credits (as a percent)	4.30%	(1.30%)	2.60%
Foreign tax rate difference (as a percent)	0.70%	0.40%	(1.20%)
Change in valuation allowance (as a percent)	(23.80%)	(22.20%)	(5.20%)
Change in tax law (as a percent)			(31.20%)
Other (as a percent)	(2.50%)	(2.90%)	0.50%
Provision for income taxes (as a percent)	0.90%	2.00%