Investments - Debt Securities At Fair Value (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt securities, at fair value	$ 23,145	$ 16,570
Corporate debt securities [Member]
Debt securities, at fair value	9,923	10,838
Securities backed by real estate [Member]
Debt securities, at fair value	2,560	2,619
Money market instruments [Member]
Debt securities, at fair value	9,332	1,681
Other [Member]
Debt securities, at fair value	$ 1,330	$ 1,432