Income Taxes - Reconciliation of Effective Income Tax Rate (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	(21.00%)	(21.00%)	(34.00%)
State income taxes, net of federal benefit	(5.80%)	(6.20%)	(8.40%)
Foreign rate differential	(0.10%)	0.00%	0.00%
Research and development tax credits	(1.10%)	(3.70%)	(1.80%)
Change in fair value of preferred stock warrant liability	0.10%	2.40%	(2.20%)
Other permanent differences	1.40%	0.90%	0.50%
Remeasurement of deferred taxes due to tax reform	0	0	0.278
Change in deferred tax asset valuation allowance	26.80%	27.80%	18.10%
Other	(0.30%)	(0.20%)	0.00%
Effective income tax rate	0.00%	0.00%	0.00%

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of reported income tax expense (benefit) in excess of (less than) expected income tax expense (benefit) computed by applying domestic federal statutory income tax rate to pretax income (loss) from continuing operations, attributable to Tax Cuts and Jobs Act.

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