Financial Instruments - Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019		Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,161,152		$ 1,694,215
Gross Unrealized Gains	4,713		9,274
Gross Unrealized Losses	(80)		(59)
Aggregate Fair Value	1,165,785		1,703,430
Cash and Cash Equivalents	437,446	[1]	818,900	$ 307,577	$ 493,982
Short-term Marketable Securities	316,361	[1]	561,472
Long-term Marketable Securities	411,978	[1]	323,058
Level 1: | Cash
Schedule of Available-for-sale Securities [Line Items]
Cash	259,347		289,755
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Aggregate Fair Value	259,347		289,755
Cash and Cash Equivalents	259,347		289,755
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		1,404,460
Gross Unrealized Gains	4,713		9,274
Gross Unrealized Losses	(80)		(59)
Aggregate Fair Value	906,438		1,413,675
Cash and Cash Equivalents	178,099		529,145
Short-term Marketable Securities	316,361		561,472
Long-term Marketable Securities	411,978		323,058
Level 2: | Money market instruments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	173,100		348,757
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Aggregate Fair Value	173,100		348,757
Cash and Cash Equivalents	173,100		348,757
Short-term Marketable Securities	0		0
Long-term Marketable Securities	0		0
Level 2: | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			192,267
Gross Unrealized Gains			4
Gross Unrealized Losses			(1)
Aggregate Fair Value			192,270
Cash and Cash Equivalents			120,662
Short-term Marketable Securities			71,608
Long-term Marketable Securities			0
Level 2: | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	518,523		542,518
Gross Unrealized Gains	3,575		6,110
Gross Unrealized Losses	(12)		(54)
Aggregate Fair Value	522,086		548,574
Cash and Cash Equivalents	0		48,728
Short-term Marketable Securities	233,294		311,690
Long-term Marketable Securities	288,792		188,156
Level 2: | U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	209,633		295,600
Gross Unrealized Gains	993		2,824
Gross Unrealized Losses	(67)		0
Aggregate Fair Value	210,559		298,424
Cash and Cash Equivalents	4,999		10,998
Short-term Marketable Securities	83,067		175,862
Long-term Marketable Securities	122,493		111,564
Level 2: | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			24,769
Gross Unrealized Gains			180
Gross Unrealized Losses			(3)
Aggregate Fair Value			24,946
Cash and Cash Equivalents			0
Short-term Marketable Securities			2,312
Long-term Marketable Securities			22,634
Level 2: | Foreign and other
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	549		549
Gross Unrealized Gains	145		156
Gross Unrealized Losses	(1)		(1)
Aggregate Fair Value	693		704
Cash and Cash Equivalents	0		0
Short-term Marketable Securities	0		0
Long-term Marketable Securities	$ 693		$ 704
Minimum
Schedule of Available-for-sale Securities [Line Items]
Long term marketable securities maturity period	1 year
Maximum
Schedule of Available-for-sale Securities [Line Items]
Short term marketable securities maturity period	1 year
Long term marketable securities maturity period	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.