Income Taxes - Difference Between U.S. Federal Corporate Tax Rate and Company's Effective Tax Rate (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Expected federal income tax rate	(21.00%)	(21.00%)	(34.00%)
Expiring credits and NOLs		1.00%
Change in valuation allowance	26.20%	37.90%	0.90%
Federal and state credits	(8.10%)	(7.40%)	(6.90%)
State income taxes, net of federal benefit	(4.70%)	(9.70%)	(3.70%)
Permanent differences	4.80%	0.50%	2.40%
Rate change related to TCJA			41.90%
Other	2.80%	(1.30%)	(0.60%)
Effective tax rate	0.00%	0.00%	0.00%
Maximum
Expected federal income tax rate			(35.00%)

Effective income tax rate reconciliation expiring credits and net operating loss carryforwards.

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Effective income tax rate reconciliation federal and state credits.

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Effective income tax rate reconciliation permanent differences.

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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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