Derivatives (Estimated Fair Value of Derivatives Assets and Liabilities after Master Netting Agreements and Cash Collateral) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Offsetting Derivative Assets [Abstract]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ 3,574	$ 3,046
Derivative Asset, Not Offset, Policy Election Deduction	(1,342)	(1,458)
Derivative Asset, Collateral, Obligation to Return Cash, Offset	(1,327)	(1,100)
Derivative Asset, Fair Value, Amount Offset Against Collateral	905	488
Derivative Asset, Collateral, Obligation to Return Securities, Offset	(840)	(487)
Derivative Asset, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	65	1
Offsetting Derivative Liabilities [Abstract]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	2,010	2,522
Derivative Liability, Not Offset, Policy Election Deduction	(1,342)	(1,458)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral	668	1,064
Derivative Liability, Collateral, Right to Reclaim Securities, Offset	(630)	(1,061)
Derivative Liability, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	$ 38	$ 3