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, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,763,780	$ 1,161,152
Gross Unrealized Gains	7,030	4,713
Gross Unrealized Losses	(15)	(80)
Aggregate Fair Value	1,770,795	1,165,785
Cash and Cash Equivalents	1,015,675	437,446	[1]
Short-term Marketable Securities	489,998	316,361	[1]
Long-term Marketable Securities	265,122	411,978	[1]
Level 1: | Cash
Schedule of Available-for-sale Securities [Line Items]
Cash	378,705	259,347
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	378,705	259,347
Cash and Cash Equivalents	378,705	259,347
Short-term Marketable Securities	0	0
Long-term Marketable Securities	0	0
Level 2:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,385,075	901,805
Gross Unrealized Gains	7,030	4,713
Gross Unrealized Losses	(15)	(80)
Aggregate Fair Value	1,392,090	906,438
Cash and Cash Equivalents	636,970	178,099
Short-term Marketable Securities	489,998	316,361
Long-term Marketable Securities	265,122	411,978
Level 2: | Money market instruments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	557,006	173,100
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Aggregate Fair Value	557,006	173,100
Cash and Cash Equivalents	557,006	173,100
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	435,954	518,523
Gross Unrealized Gains	4,558	3,575
Gross Unrealized Losses	(14)	(12)
Aggregate Fair Value	440,498	522,086
Cash and Cash Equivalents	0	0
Short-term Marketable Securities	280,000	233,294
Long-term Marketable Securities	160,498	288,792
Level 2: | U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	361,336	209,633
Gross Unrealized Gains	2,182	993
Gross Unrealized Losses	(1)	(67)
Aggregate Fair Value	363,517	210,559
Cash and Cash Equivalents	79,964	4,999
Short-term Marketable Securities	199,559	83,067
Long-term Marketable Securities	83,994	122,493
Level 2: | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,430
Gross Unrealized Gains	129
Gross Unrealized Losses	0
Aggregate Fair Value	23,559
Cash and Cash Equivalents	0
Short-term Marketable Securities	3,639
Long-term Marketable Securities	19,920
Level 2: | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,800
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Aggregate Fair Value	6,800
Cash and Cash Equivalents	0
Short-term Marketable Securities	6,800
Long-term Marketable Securities	0
Level 2: | Foreign and other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	549	549
Gross Unrealized Gains	161	145
Gross Unrealized Losses	0	(1)
Aggregate Fair Value	710	693
Cash and Cash Equivalents	0	0
Short-term Marketable Securities	0	0
Long-term Marketable Securities	$ 710	$ 693
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]
Short term marketable securities maturity period	1 year	1 year
Long term marketable securities maturity period	5 years	5 years

[1] December 31, 2019 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, 2019, filed with the SEC on February 27, 2020.