Financial Instruments - Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018		Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,147,834	$ 1,322,661
Gross Unrealized Gains	4,928	497
Gross Unrealized Losses	(165)	(2,986)
Aggregate Fair Value	1,152,596	1,320,172
Cash and Cash Equivalents	423,220	493,982	[1]	$ 882,184	$ 598,028
Short-term Marketable Securities	297,572	590,326	[1]
Long-term Marketable Securities	431,804	235,864	[1]
Level 1: | Cash
Schedule of Available-for-sale Securities [Line Items]
Cash	294,543	228,809
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	294,543	228,809
Cash and Cash Equivalents	294,543	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	853,291	1,093,852
Gross Unrealized Gains	4,928	497
Gross Unrealized Losses	(165)	(2,986)
Aggregate Fair Value	858,054	1,091,363
Cash and Cash Equivalents	128,678	265,173
Short-term Marketable Securities	297,572	590,326
Long-term Marketable Securities	431,804	235,864
Level 2: | Money market instruments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	128,678	205,736
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	128,678	205,736
Cash and Cash Equivalents	128,678	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	519,693	564,852
Gross Unrealized Gains	3,713	214
Gross Unrealized Losses	(67)	(2,288)
Aggregate Fair Value	523,339	562,778
Cash and Cash Equivalents	0	2,000
Short-term Marketable Securities	206,782	376,545
Long-term Marketable Securities	316,557	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
Level 2: | U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	204,371	240,436
Gross Unrealized Gains	1,073	144
Gross Unrealized Losses	(97)	(697)
Aggregate Fair Value	205,347	239,883
Cash and Cash Equivalents	0	31,474
Short-term Marketable Securities	90,790	156,967
Long-term Marketable Securities	114,557	51,442
Level 2: | Foreign and other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	549	5,126
Gross Unrealized Gains	142	139
Gross Unrealized Losses	(1)	(1)
Aggregate Fair Value	690	5,264
Cash and Cash Equivalents	0	3,999
Short-term Marketable Securities	0	1,076
Long-term Marketable Securities	$ 690	$ 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.