Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 166,145	$ 296,536
Gross Unrealized Gains	2
Gross Unrealized Losses	(315)	(409)
Fair Value	165,832	296,127
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,938	22,436
Fair Value	12,938	22,436
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,310	121,470
Gross Unrealized Gains	2
Gross Unrealized Losses	(89)	(136)
Fair Value	64,223	121,334
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	88,897	152,630
Gross Unrealized Losses	(226)	(273)
Fair Value	$ 88,671	$ 152,357

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).

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-Publisher FASB
-Name Accounting Standards Codification
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