Income Taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal income tax at statutory rate	34.00%	34.00%	34.00%
State taxes (net of federal benefit)	2.40%	2.49%	3.86%
Permanent differences	(15.03%)	(8.21%)	(2.94%)
Federal research and development credit	7.08%	2.06%	2.42%
Change in valuation allowance	(27.79%)	(30.19%)	(37.36%)
Nondeductible expenses and others	(0.66%)	(0.15%)	0.02%
Change in valuation allowance for Tax Act impact	84.37%	0.00%	0.00%
Change in deferred balance before valuation allowance for Tax Reform impact	(84.37%)	0.00%	0.00%
Effective income tax rate	0.00%	0.00%	0.00%

Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Valuation Allowance, Percent

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

+ References

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-Publisher FASB
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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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