Basis of Presentation and Accounting Policies (Change in Accounting Principle - Consolidated Statements of Comprehensive Income) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020		Jun. 30, 2021	Jun. 30, 2020
Other noninterest expenses	$ 10	$ 19		$ 9	$ 36
Provision for income taxes	93	28	[1]	191	9	[1]
Net income	$ 328	$ 118	[1]	$ 678	$ 59	[1]
Basic earnings per common share	$ 2.35	$ 0.85		$ 4.81	$ 0.42
Diluted earnings per common share	$ 2.32	$ 0.84		$ 4.76	$ 0.42
Accounting change impact
Other noninterest expenses		$ (6)		$ (9)	$ (14)
Provision for income taxes	$ 1	1		2	3
Net income	$ 4	$ 5		$ 7	$ 11
Basic earnings per common share	$ 0.03	$ 0.04		$ 0.05	$ 0.08
Diluted earnings per common share	$ 0.03	$ 0.04		$ 0.05	$ 0.08
Previously Reported
Other noninterest expenses		$ 25			$ 50
Provision for income taxes		27			6
Net income		$ 113			$ 48
Basic earnings per common share		$ 0.81			$ 0.34
Diluted earnings per common share		$ 0.80			$ 0.34

[1] See Defined Benefit Pension and Other Postretirement Costs in Note 1.