Investments - (Summary of Allowance for Credit Losses on HTM Investments) (Detail) - Variable Interest Entity Primary Beneficiary [Member] - Accounting Standards Update 2016-13 [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Beginning balance	$ 23	$ 37	[1]
Current period provision for expected credit losses	(23)	(37)
Initial allowance recognized for PCD assets	0	0
Write- Offs	0	0
Recoveries	0	0
Ending balance	0	0
Corporate Obligations [Member]
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]
Beginning balance	23	37	[1]
Current period provision for expected credit losses	(23)	(37)
Initial allowance recognized for PCD assets	0	0
Write- Offs	0	0
Recoveries	0	0
Ending balance	$ 0	$ 0

[1] Represents transition adjustment upon adoption of ASU 2016-13.