Securities - Pre-Tax Net Securities Gains (Losses) by Type (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains	$ 2	$ 2	$ 9	$ 4	$ 27
U.S. Treasury
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains	0	0	1	(4)	7
Foreign government agencies
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains	0	0	5	1	7
Supranational
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains	0	0	0	0	6
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains	$ 2	$ 2	$ 3	$ 7	$ 7