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Fair Value Measurements and Marketable Securities - Summary of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 35,110	$ 61,333
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(6)	(58)
Fair Value	35,104	61,275
Corporate Bonds (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,126	50,892
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	(48)
Fair Value	12,121	50,844
Commercial Paper (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,001	9,961
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(10)
Fair Value	21,000	9,951
U.S. Government Securities (Due within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,983
Gross Unrealized Gains	0
Fair Value	$ 1,983
Certificates of Deposit (Due Within 1 Year) [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		480
Gross Unrealized Gains		0
Fair Value		$ 480

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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