Investments - Debt Securities At Fair Value (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt securities, at fair value	$ 16,533	$ 16,570
Corporate debt securities [Member]
Debt securities, at fair value	10,720	10,838
Securities backed by real estate [Member]
Debt securities, at fair value	1,870	2,619
Money market instruments [Member]
Debt securities, at fair value	2,197	1,681
Other [Member]
Debt securities, at fair value	$ 1,746	$ 1,432