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Investments - Summary of Investments Classified as AFS Debt Securities (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	$ 135,197	$ 94,572
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	414,951	544,978
Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	158,170	85,677
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	502,660	510,889
Gross Unrealized Gains	894	8
Gross Unrealized Losses	(29)	(1,698)
Aggregate Estimated Fair Value	503,525	509,199
Short-term investments | Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	135,156	94,617
Gross Unrealized Gains	45
Gross Unrealized Losses	(4)	(45)
Aggregate Estimated Fair Value	$ 135,197	$ 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	$ 254,811	$ 395,385
Gross Unrealized Gains	577
Gross Unrealized Losses	(21)	(1,598)
Aggregate Estimated Fair Value	$ 255,367	$ 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	$ 112,693	$ 20,887
Gross Unrealized Gains	272	8
Gross Unrealized Losses	(4)	(55)
Aggregate Estimated Fair Value	$ 112,961	$ 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	$ 204,052	$ 216,270
Gross Unrealized Gains	794	228
Gross Unrealized Losses	(53)	(470)
Aggregate Estimated Fair Value	204,793	216,028
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	159,035	151,594
Gross Unrealized Gains	602	66
Gross Unrealized Losses	(53)	(469)
Aggregate Estimated Fair Value	$ 159,584	$ 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	$ 45,017	$ 64,676
Gross Unrealized Gains	192	162
Gross Unrealized Losses		(1)
Aggregate Estimated Fair Value	$ 45,209	$ 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

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