Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,161,152	$ 1,322,661
Gross Unrealized Gains	4,713	497
Gross Unrealized Losses	(80)	(2,986)
Aggregate Fair Value	1,165,785	1,320,172
Cash and Cash Equivalents	437,446	493,982
Short-term Marketable Securities	316,361	590,326
Long-term Marketable Securities	411,978	235,864
Quoted Price in Active Markets For Identical Assets (Level 1) \| Cash
Schedule of Available-for-sale Securities [Line Items]
Cash	259,347	228,809
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	259,347	228,809
Cash and Cash Equivalents	259,347	228,809
Short-term Marketable Securities	0	0
Long-term Marketable Securities	0	0
Level 2
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	901,805	1,093,852
Gross Unrealized Gains	4,713	497
Gross Unrealized Losses	(80)	(2,986)
Aggregate Fair Value	906,438	1,091,363
Cash and Cash Equivalents	178,099	265,173
Short-term Marketable Securities	316,361	590,326
Long-term Marketable Securities	411,978	235,864
Level 2 \| Money Market Instruments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	173,100	205,736
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	173,100	205,736
Cash and Cash Equivalents	173,100	205,736
Short-term Marketable Securities	0	0
Long-term Marketable Securities	0	0
Level 2 \| Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	518,523	564,852
Gross Unrealized Gains	3,575	214
Gross Unrealized Losses	(12)	(2,288)
Aggregate Fair Value	522,086	562,778
Cash and Cash Equivalents	0	2,000
Short-term Marketable Securities	233,294	376,545
Long-term Marketable Securities	288,792	184,233
Level 2 \| Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		77,702
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Aggregate Fair Value		77,702
Cash and Cash Equivalents		21,964
Short-term Marketable Securities		55,738
Long-term Marketable Securities		0
Level 2 \| U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	209,633	240,436
Gross Unrealized Gains	993	144
Gross Unrealized Losses	(67)	(697)
Aggregate Fair Value	210,559	239,883
Cash and Cash Equivalents	4,999	31,474
Short-term Marketable Securities	83,067	156,967
Long-term Marketable Securities	122,493	51,442
Level 2 \| Foreign and Other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	549	5,126
Gross Unrealized Gains	145	139
Gross Unrealized Losses	(1)	(1)
Aggregate Fair Value	693	5,264
Cash and Cash Equivalents	0	3,999
Short-term Marketable Securities	0	1,076
Long-term Marketable Securities	$ 693	$ 189

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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).

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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).

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