Income Taxes - Reconciliation of U.S. Statutory Income Tax Rate to Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax effected at statutory rate	21.00%	21.00%	34.00%
State taxes, net of federal benefit	7.90%	6.30%	8.50%
Non-deductible items	1.60%	0.00%	(1.30%)
Change in valuation allowance	(33.00%)	(28.50%)	(20.40%)
Federal research and development credits	2.50%	1.50%	4.80%
Foreign tax rate differential	(0.20%)	(0.20%)	(1.10%)
Impact of federal rate change on net deferred taxes	0.00%	0.00%	(25.00%)
Other	0.20%	(0.20%)	0.50%
Effective tax rate	0.00%	(0.10%)	0.00%