Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 766,748	$ 212,642
Gross Unrealized Gains	6	2
Gross Unrealized Losses	(151)	(31)
Fair Value	766,603	212,613
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	225,363	49,612
Gross Unrealized Gains	5
Gross Unrealized Losses	(33)	(6)
Fair Value	225,335	49,606
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	110,454	48,982
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(92)	(25)
Fair Value	110,363	48,959
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,518
Gross Unrealized Losses	(15)
Fair Value	26,503
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	211,150	65,583
Gross Unrealized Losses	(6)
Fair Value	211,144	65,583
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	193,263	48,465
Gross Unrealized Losses	(5)
Fair Value	$ 193,258	$ 48,465

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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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).

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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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