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Investments - Summary of Investments Classified as AFS Debt Securities (Detail) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		$ 702,861	$ 725,227
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		116,490	94,572
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		419,293	544,978
Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		89,713	85,677
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		478,553	510,889
Gross Unrealized Gains	[1]	543	8
Gross Unrealized Losses	[1]	(103)	(1,698)
Aggregate Estimated Fair Value		478,993	509,199
Short-term investments | Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		116,446	94,617
Gross Unrealized Gains	[1]	45
Gross Unrealized Losses	[1]	(1)	(45)
Aggregate Estimated Fair Value		$ 116,490	$ 94,572
Short-term investments | Commercial paper | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Short-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 302,876	$ 395,385
Gross Unrealized Gains	[1]	299
Gross Unrealized Losses	[1]	(96)	(1,598)
Aggregate Estimated Fair Value		$ 303,079	$ 393,787
Short-term investments | Corporate debt securities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Short-term investments | Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 59,231	$ 20,887
Gross Unrealized Gains	[1]	199	8
Gross Unrealized Losses	[1]	(6)	(55)
Aggregate Estimated Fair Value		$ 59,424	$ 20,840
Short-term investments | Securities of government-sponsored entities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 145,418	$ 216,270
Gross Unrealized Gains	[1]	1,101	228
Gross Unrealized Losses	[1]	(16)	(470)
Aggregate Estimated Fair Value		146,503	216,028
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		115,454	151,594
Gross Unrealized Gains	[1]	776	66
Gross Unrealized Losses	[1]	(16)	(469)
Aggregate Estimated Fair Value		$ 116,214	$ 151,191
Long-term investments | Corporate debt securities | Minimum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments | Corporate debt securities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		2 years	2 years
Long-term investments | Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 29,964	$ 64,676
Gross Unrealized Gains	[1]	325	162
Gross Unrealized Losses	[1]		(1)
Aggregate Estimated Fair Value		$ 30,289	$ 64,837
Long-term investments | Securities of government-sponsored entities | Minimum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments | Securities of government-sponsored entities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		2 years	2 years
[1] Unrealized gains and losses, net of tax, are included in other comprehensive income (loss)

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