Accumulated Other Comprehensive Income (Accumulated Other Comprehensive Income Balances) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Accumulated Other Comprehensive Income
Beginning Balance	$ (252)	$ 88	[1]	$ (252)		$ (152)
Other net changes		5,306		340		(67)
Ending Balance		5,394	[1]	88	[1]	(252)
Adoption of New Accounting Standards [Member]
Accumulated Other Comprehensive Income
Other net changes						(33)
Net unrealized gain (loss), excluding transfers to available for sale from held to maturity [Member]
Accumulated Other Comprehensive Income
Other net changes		4,246		309		(93)
Other reclassifications included in other revenue [Member]
Accumulated Other Comprehensive Income
Other net changes		(3)		(5)
Amortization of amounts previously recorded upon transfer to held to maturity from available for sale [Member]
Accumulated Other Comprehensive Income
Other net changes				27		27
Net unrealized gain (loss) on securities transferred to available for sale from held to maturity [Member]
Accumulated Other Comprehensive Income
Other net changes	$ 19	1,057		19
Other [Member]
Accumulated Other Comprehensive Income
Other net changes		$ 6		$ (10)		$ (1)

[1] Certain prior year amounts have been reclassified to conform to the current year presentation. See Note 1 for additional information.