Marketable Debt Securities - Summary of Available-for-Sale Short-Term Investments Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 893,603	$ 36,230
Gross Unrealized Gains	148	2
Gross Unrealized Losses	(19)
Aggregate Fair Value	893,732	36,232
Long-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	175,150
Gross Unrealized Gains	151
Gross Unrealized Losses	(1)
Aggregate Fair Value	175,300
Corporate Debt Securities [Member] | Short-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	814,817	36,230
Gross Unrealized Gains	148	2
Gross Unrealized Losses	(19)
Aggregate Fair Value	814,946	$ 36,232
Corporate Debt Securities [Member] | Long-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	108,171
Gross Unrealized Gains	118
Gross Unrealized Losses	(1)
Aggregate Fair Value	108,288
U.S. Goverment and Goverment Agency Debt Securities [Member] | Short-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	25,000
Aggregate Fair Value	25,000
U.S. Goverment and Goverment Agency Debt Securities [Member] | Long-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	66,979
Gross Unrealized Gains	33
Aggregate Fair Value	67,012
Foreign Certificates of Deposit and Time Deposits [Member] | Short-term Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	53,786
Aggregate Fair Value	$ 53,786