Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	$ 1,054
Continuous unrealized loss position, less than 12 months, gross unrealized losses	(2)
Continuous unrealized loss position, 12 months or greater, fair value	0
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	0
Continuous unrealized loss position, fair value	1,054	$ 637
Continuous unrealized loss position, gross unrealized losses	(2)
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	1,027
Continuous unrealized loss position, less than 12 months, gross unrealized losses	(1)
Continuous unrealized loss position, 12 months or greater, fair value	0
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	0
Continuous unrealized loss position, fair value	1,027
Continuous unrealized loss position, gross unrealized losses	(1)
Mortgage and asset backed securities
Debt Securities, Available-for-sale [Line Items]
Continuous unrealized loss position, less than 12 months, fair value	27
Continuous unrealized loss position, less than 12 months, gross unrealized losses	(1)
Continuous unrealized loss position, 12 months or greater, fair value	0
Continuous unrealized loss position, 12 months or greater, gross unrealized loss	0
Continuous unrealized loss position, fair value	27
Continuous unrealized loss position, gross unrealized losses	$ (1)