Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		$ 591,706	$ 508,578
Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		37,157	75,362
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		543,648	414,815
Domestic and foreign government agencies debt securities
Schedule Of Available For Sale Securities [Line Items]
Aggregate Estimated Fair Value		10,901	18,401
Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		343,662	261,675
Gross Unrealized Gains	[1]	0	1
Gross Unrealized Losses	[1]	(1,344)	(459)
Aggregate Estimated Fair Value		342,318	261,217
Short-term investments | Commercial paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		37,211	75,396
Gross Unrealized Gains	[1]	0	1
Gross Unrealized Losses	[1]	(54)	(35)
Aggregate Estimated Fair Value		$ 37,157	$ 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		$ 300,451	$ 178,776
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(1,243)	(400)
Aggregate Estimated Fair Value		$ 299,208	$ 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		$ 6,000	$ 7,503
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(47)	(24)
Aggregate Estimated Fair Value		$ 5,953	$ 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		$ 250,853	$ 248,753
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(1,465)	(1,392)
Aggregate Estimated Fair Value		249,388	247,361
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		245,850	237,749
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(1,410)	(1,310)
Aggregate Estimated Fair Value		$ 244,440	$ 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		$ 5,003	$ 11,004
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(55)	(82)
Aggregate Estimated Fair Value		$ 4,948	$ 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 loss.