Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 114,216	$ 83,251
Gross Unrealized Gains	3
Gross Unrealized Losses	(60)	(70)
Estimated Fair Value	114,159	83,181
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,195	39,988
Gross Unrealized Losses	(57)	(37)
Estimated Fair Value	68,138	39,951
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,495
Estimated Fair Value	1,495
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,096	7,799
Gross Unrealized Losses		(5)
Estimated Fair Value	1,096	7,794
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,735	15,919
Gross Unrealized Losses	(1)	(12)
Estimated Fair Value	6,734	15,907
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,695	19,545
Gross Unrealized Gains	3
Gross Unrealized Losses	(2)	(16)
Estimated Fair Value	$ 36,696	$ 19,529

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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-Topic 320
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-Section 50
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Amount, before tax, of unrealized loss 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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