Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	$ 725,227	$ 508,578
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	94,572	75,362
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	544,978	414,815
Securities of government-sponsored entities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value	85,677	18,401
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	510,889	261,675
Gross Unrealized Gains	8	1
Gross Unrealized Losses	(1,698)	(459)
Aggregate Estimated Fair Value	509,199	261,217
Short-term investments \| Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	94,617	75,396
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(45)	(35)
Aggregate Estimated Fair Value	$ 94,572	$ 75,362
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	$ 395,385	$ 178,776
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,598)	(400)
Aggregate Estimated Fair Value	$ 393,787	$ 178,376
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	$ 20,887	$ 7,503
Gross Unrealized Gains	8	0
Gross Unrealized Losses	(55)	(24)
Aggregate Estimated Fair Value	$ 20,840	$ 7,479
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	$ 216,270	$ 248,753
Gross Unrealized Gains	228	0
Gross Unrealized Losses	(470)	(1,392)
Aggregate Estimated Fair Value	216,028	247,361
Long-term investments \| Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	151,594	237,749
Gross Unrealized Gains	66	0
Gross Unrealized Losses	(469)	(1,310)
Aggregate Estimated Fair Value	$ 151,191	$ 236,439
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	$ 64,676	$ 11,004
Gross Unrealized Gains	162	0
Gross Unrealized Losses	(1)	(82)
Aggregate Estimated Fair Value	$ 64,837	$ 10,922
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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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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