Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
Research and development tax credits	(7.70%)	(8.80%)	(5.70%)
State taxes, net of federal benefit	(4.80%)	(5.80%)	(5.30%)
Stock-based compensation	1.20%	(6.30%)	4.10%
Revaluation of preferred stock warrant liability	33.10%
Other	(0.30%)	0.10%	0.30%
Change in deferred tax asset valuation allowance	12.50%	54.80%	40.60%
Effective income tax rate	0.00%	0.00%	0.00%

Effective income tax rate reconciliation revaluation of preferred stock warrants liability.

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- Definition

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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-Subparagraph (SX 210.4-08.(h)(2))
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
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-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

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-Name Accounting Standards Codification
-Topic 740
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-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

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-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

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-Name Accounting Standards Codification
-Topic 740
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Percentage of the difference, between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, that is attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority interest income (expense), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, change in enacted tax rate, prior year income taxes, change in deferred tax asset valuation allowance, and other adjustments.

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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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