Financial Instruments - Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018		Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,101,824	$ 1,322,661
Gross Unrealized Gains	4,602	497
Gross Unrealized Losses	(358)	(2,986)
Aggregate Fair Value	1,106,068	1,320,172
Cash and Cash Equivalents	307,577	493,982	[1]	$ 427,411	$ 598,028
Short-term Marketable Securities	423,526	590,326	[1]
Long-term Marketable Securities	374,965	235,864	[1]
Level 1: | Cash
Schedule of Available-for-sale Securities [Line Items]
Cash	226,959	228,809
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	226,959	228,809
Cash and Cash Equivalents	226,959	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	874,865	1,093,852
Gross Unrealized Gains	4,602	497
Gross Unrealized Losses	(358)	(2,986)
Aggregate Fair Value	879,109	1,091,363
Cash and Cash Equivalents	80,618	265,173
Short-term Marketable Securities	423,526	590,326
Long-term Marketable Securities	374,965	235,864
Level 2: | Money market instruments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,648	205,736
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	68,648	205,736
Cash and Cash Equivalents	68,648	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	564,624	564,852
Gross Unrealized Gains	3,270	214
Gross Unrealized Losses	(216)	(2,288)
Aggregate Fair Value	567,678	562,778
Cash and Cash Equivalents	0	2,000
Short-term Marketable Securities	279,033	376,545
Long-term Marketable Securities	288,645	184,233
Level 2: | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,972	77,702
Gross Unrealized Gains	0	0
Gross Unrealized Losses		0
Aggregate Fair Value	16,972	77,702
Cash and Cash Equivalents	8,981	21,964
Short-term Marketable Securities	7,991	55,738
Long-term Marketable Securities	0
Level 2: | U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	224,071	240,436
Gross Unrealized Gains	1,185	144
Gross Unrealized Losses	(141)	(697)
Aggregate Fair Value	225,115	239,883
Cash and Cash Equivalents	2,989	31,474
Short-term Marketable Securities	136,502	156,967
Long-term Marketable Securities	85,624	51,442
Level 2: | Foreign and other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	550	5,126
Gross Unrealized Gains	147	139
Gross Unrealized Losses	(1)	(1)
Aggregate Fair Value	696	5,264
Cash and Cash Equivalents	0	3,999
Short-term Marketable Securities	0	1,076
Long-term Marketable Securities	$ 696	$ 189
Minimum
Schedule of Available-for-sale Securities [Line Items]
Long term marketable securities maturity period	1 year	1 year
Maximum
Schedule of Available-for-sale Securities [Line Items]
Long term marketable securities maturity period	5 years	5 years
Short term marketable securities maturity period	1 year	1 year

[1] December 31, 2018 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, 2018, filed with the SEC on February 28, 2019.