Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Percent of pre-tax income:
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%	34.00%
Permanent Differences		0.00%	(0.10%)	0.10%
State taxes, net of federal benefit		8.30%	7.90%	6.70%
Research and development credit		3.40%	2.80%	1.70%
Rate change due to the Tax Act		0.00%	0.00%	(58.50%)
Withholding Tax		0.00%	(2.40%)	0.00%
Other		3.30%	0.00%	1.00%
Change in valuation allowance		(36.00%)	(31.60%)	15.00%
Effective income tax rate		0.00%	(2.40%)	0.00%
U.S. federal
Operating loss carryforwards
Net operating loss carryforwards		$ 76.7
State Jurisdiction
Operating loss carryforwards
Tax credit carryforwards		75.2
Research Tax Credit Carryforward | U.S. federal
Operating loss carryforwards
Tax credit carryforwards		$ 14.1