Marketable Securities - Summary of Marketable Securities Held (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Equity Securities, Amortized cost / Cost	$ 20,017	$ 20,017
Equity Securities, Unrealized gains		2,150
Equity Securities, Unrealized losses	(934)
Equity Securities, Fair value	19,083	22,167
Marketable Securities, Amortized cost / Cost	1,509,696	1,488,746
Marketable Securities, Unrealized gains	1,685	3,113
Marketable Securities, Unrealized losses	(2,353)	(3,011)
Marketable Securities, Fair value	1,509,028	1,488,848
U.S. Government Agency Securities and Treasuries [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	1,357,641	1,459,649
Debt Securities, Unrealized gains	1,604	963
Debt Securities, Unrealized losses	(1,400)	(3,011)
Debt Securities, Fair value	1,357,845	1,457,601
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	5,960	9,080
Debt Securities, Fair value	5,960	$ 9,080
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	83,536
Debt Securities, Unrealized gains	81
Debt Securities, Unrealized losses	(19)
Debt Securities, Fair value	83,598
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Debt Securities. Amortized cost / Cost	42,542
Debt Securities, Fair value	$ 42,542

Equity securities unrealized gain.

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Equity securities unrealized loss.

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Marketable securities amortized cost.

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Marketable securities unrealized gains.

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Marketable securities unrealized loss.

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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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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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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Amount of investment in marketable security.

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