Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017		Jun. 30, 2017	Dec. 31, 2016
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		$ 1,647,842	$ 1,786,045
Gross Unrealized Gains		291	300
Gross Unrealized Losses		(5,072)	(4,592)
Fair Value		1,643,061	1,781,753
Cash and Cash Equivalents		427,411	598,028	[1]	$ 354,864	$ 408,330
Short-term Marketable Securities	[2]	935,662	797,940
Long-term Marketable Securities	[3]	279,988	385,785
Level 1 | Cash
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash		209,062	340,253
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Cash and Cash Equivalents		209,062	340,253
Short-term Marketable Securities	[2]	0	0
Long-term Marketable Securities	[3]	0	0
Level 2
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		1,438,780	1,445,792
Gross Unrealized Gains		291	300
Gross Unrealized Losses		(5,072)	(4,592)
Fair Value		1,433,999	1,441,500
Cash and Cash Equivalents		218,349	257,775
Short-term Marketable Securities	[2]	935,662	797,940
Long-term Marketable Securities	[3]	279,988	385,785
Level 2 | Money Market Instruments
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		177,882	215,441
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		177,882	215,441
Cash and Cash Equivalents		177,882	215,441
Short-term Marketable Securities	[2]	0	0
Long-term Marketable Securities	[3]	0	0
Level 2 | Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		643,979	707,652
Gross Unrealized Gains		136	150
Gross Unrealized Losses		(3,452)	(2,553)
Fair Value		640,663	705,249
Cash and Cash Equivalents		7,708	3,096
Short-term Marketable Securities	[2]	411,185	406,188
Long-term Marketable Securities	[3]	221,770	295,965
Level 2 | Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		51,818	24,566
Gross Unrealized Gains		1	0
Gross Unrealized Losses		0	0
Fair Value		51,819	24,566
Cash and Cash Equivalents		18,947	2,751
Short-term Marketable Securities	[2]	32,871	21,815
Long-term Marketable Securities	[3]	0	0
Level 2 | U.S. Government Agency Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		532,066	472,593
Gross Unrealized Gains		4	0
Gross Unrealized Losses		(1,597)	(1,975)
Fair Value		530,473	470,618
Cash and Cash Equivalents		11,312	35,497
Short-term Marketable Securities	[2]	461,143	345,501
Long-term Marketable Securities	[3]	58,019	89,620
Level 2 | Foreign and Other
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		33,035	25,540
Gross Unrealized Gains		150	150
Gross Unrealized Losses		(23)	(64)
Fair Value		33,162	25,626
Cash and Cash Equivalents		2,500	990
Short-term Marketable Securities	[2]	30,463	24,436
Long-term Marketable Securities	[3]	$ 199	$ 200

[1] December 31, 2017 balances were derived from the audited Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the SEC on February 26, 2018.

[2] The Company’s short-term marketable securities mature in one year or less.

[3] The Company’s long-term marketable securities mature between one and five years.