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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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An excess of the fair value of the modified award over the fair value of the award immediately before the modification.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 718
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Cash Equivalents Investments Maturity Period

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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash equivalents, highly-liquid investments maturity (or less)	3 months
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Property and equipment, estimated useful lives	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Property and equipment, estimated useful lives	5 years
ASU 2016-09, Forfeiture Rate Component \| Additional Paid-In Capital
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative effect of new accounting principle		$ 50
ASU 2016-09, Forfeiture Rate Component \| Accumulated Deficit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative effect of new accounting principle		$ (50)
ASU 2017-09
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option modifications	$ 724