Amortized Cost, Gross Unrealized Gain (Loss) Positions and Estimated Fair Value for Available-For-Sale Debt Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	$ 857,899	$ 725,227
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	144,472	94,572
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	521,946	544,978
Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	191,481	85,677
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	557,296	510,889
Gross Unrealized Gains	1,006	8
Gross Unrealized Losses	(57)	(1,698)
Aggregate Estimated Fair Value	558,245	509,199
Short-term investments \| Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	144,460	94,617
Gross Unrealized Gains	27	0
Gross Unrealized Losses	(15)	(45)
Aggregate Estimated Fair Value	$ 144,472	$ 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	$ 270,485	$ 395,385
Gross Unrealized Gains	557	0
Gross Unrealized Losses	(42)	(1,598)
Aggregate Estimated Fair Value	$ 271,000	$ 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	$ 142,351	$ 20,887
Gross Unrealized Gains	422	8
Gross Unrealized Losses		(55)
Aggregate Estimated Fair Value	$ 142,773	$ 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	$ 299,190	$ 216,270
Gross Unrealized Gains	534	228
Gross Unrealized Losses	(70)	(470)
Aggregate Estimated Fair Value	299,654	216,028
Long-term investments \| Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	250,499	151,594
Gross Unrealized Gains	515	66
Gross Unrealized Losses	(68)	(469)
Aggregate Estimated Fair Value	$ 250,946	$ 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	$ 48,691	$ 64,676
Gross Unrealized Gains	19	162
Gross Unrealized Losses	(2)	(1)
Aggregate Estimated Fair Value	$ 48,708	$ 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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- Definition

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).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

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).

+ References

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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