Marketable Securities - Summary of Available-for-Sale Short-Term and Long-Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 55,590	$ 893,603
Gross Unrealized Gains	291	148
Gross Unrealized Losses	(2)	(19)
Aggregate Fair Value	55,879	893,732
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,540	175,150
Gross Unrealized Gains		151
Gross Unrealized Losses		(1)
Aggregate Fair Value	2,540	175,300
Corporate Debt Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,743	814,817
Gross Unrealized Gains	291	148
Gross Unrealized Losses	(2)	(19)
Aggregate Fair Value	51,032	814,946
Corporate Debt Securities [Member] | Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,540	108,171
Gross Unrealized Gains		118
Gross Unrealized Losses		(1)
Aggregate Fair Value	2,540	108,288
Foreign Certificates of Deposit and Time Deposits [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,847	53,786
Aggregate Fair Value	$ 4,847	53,786
U S Goverment And Goverment Agency Debt Securities [Member] | Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		25,000
Aggregate Fair Value		25,000
U S Goverment And Goverment Agency Debt Securities [Member] | Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		66,979
Gross Unrealized Gains		33
Aggregate Fair Value		$ 67,012