Available-For-Sale Debt Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Investments [Line Items]
Marketable securities	$ 857,899	$ 725,227
Commercial paper
Schedule Of Investments [Line Items]
Marketable securities	144,472	94,572
Corporate debt securities
Schedule Of Investments [Line Items]
Marketable securities	521,946	544,978
Securities of government-sponsored entities
Schedule Of Investments [Line Items]
Marketable securities	$ 191,481	$ 85,677