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Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		$ 624,866	$ 508,578
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		42,465	75,362
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		541,694	414,815
Domestic and foreign government agencies debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		40,707	18,401
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		441,141	261,675
Gross Unrealized Gains	[1]	6	1
Gross Unrealized Losses	[1]	(1,538)	(459)
Aggregate Estimated Fair Value		439,609	261,217
Short-term investments | Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		42,514	75,396
Gross Unrealized Gains	[1]	0	1
Gross Unrealized Losses	[1]	(49)	(35)
Aggregate Estimated Fair Value		$ 42,465	$ 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		$ 387,627	$ 178,776
Gross Unrealized Gains	[1]	6	0
Gross Unrealized Losses	[1]	(1,407)	(400)
Aggregate Estimated Fair Value		$ 386,226	$ 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 | Domestic and foreign government agencies debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 11,000	$ 7,503
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(82)	(24)
Aggregate Estimated Fair Value		$ 10,918	$ 7,479
Short-term investments | Domestic and foreign government agencies debt securities | 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		$ 185,781	$ 248,753
Gross Unrealized Gains	[1]	1	0
Gross Unrealized Losses	[1]	(525)	(1,392)
Aggregate Estimated Fair Value		185,257	247,361
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		155,935	237,749
Gross Unrealized Gains	[1]	1	0
Gross Unrealized Losses	[1]	(468)	(1,310)
Aggregate Estimated Fair Value		$ 155,468	$ 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 | Domestic and foreign government agencies debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		$ 29,846	$ 11,004
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(57)	(82)
Aggregate Estimated Fair Value		$ 29,789	$ 10,922
Long-term investments | Domestic and foreign government agencies debt securities | Minimum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments | Domestic and foreign government agencies debt securities | Maximum
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities contractual maturity		2 years	2 years
[1] Unrealized gains and losses are included in other comprehensive income (loss).

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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