Securities - Pre-Tax Net Securities Gains (Losses) by Type (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	$ 9	$ 9	$ (1)	$ 27	$ 7
Foreign government agencies
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	5	2	0	7	0
U.S. Treasury
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	1	1	0	7	4
Supranational
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	0	6	0	6	0
Sovereign debt/sovereign guaranteed
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	1	2	0	3	3
State and political subdivisions
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	0	0	0	0	2
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total net securities gains (losses)	$ 2	$ (2)	$ (1)	$ 4	$ (2)