Income Taxes - Tax Reconciliation (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	21.00%	35.00%	35.00%
State, net of federal benefit	4.00%	3.00%	2.80%
Permanent items	0.10%	(1.10%)	(0.50%)
Research and development ("R&D") tax credit	1.30%	1.20%	1.30%
Federal tax rate change	0.00%	(92.60%)	0.00%
Other	(0.10%)	1.10%	7.10%
Change in valuation allowance	(26.30%)	53.40%	(45.70%)
Total	0.00%	0.00%	0.00%

Effective Income Tax Rate Reconciliation, Permanent Items, Percent

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Effective Income Tax Rate Reconciliation, Tax Cuts And Jobs Act Of 2017, Percent

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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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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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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