Short-Term Borrowings - Outstanding and weighted-average interest rates of short-term borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Weighted-average interest rate as of year-end	0.93%	1.64%
Securities Sold Under Repurchase Agreements
Short-term Debt [Line Items]
Balance as of December 31	$ 3,413	$ 1,102	$ 1,082
Maximum outstanding as of any month-end	5,373	4,125	3,441
Average outstanding during the year	$ 2,615	$ 1,616	$ 2,048
Weighted-average interest rate as of year-end	0.00%	0.00%	1.38%
Weighted-average interest rate during the year	0.14%	1.90%	0.62%
Tax-Exempt Investment Program
Short-term Debt [Line Items]
Balance as of December 31	$ 616	$ 823	$ 931
Maximum outstanding as of any month-end	823	931	1,078
Average outstanding during the year	$ 771	$ 898	$ 1,023
Weighted-average interest rate as of year-end	0.23%	1.75%	1.74%
Weighted-average interest rate during the year	0.78%	1.51%	1.46%
Other
Short-term Debt [Line Items]
Balance as of December 31	$ 3,302	$ 0	$ 2,000
Maximum outstanding as of any month-end	25,665	0	$ 2,000
Average outstanding during the year	$ 8,251	$ 3
Weighted-average interest rate as of year-end	1.35%	0.00%	2.68%
Weighted-average interest rate during the year	1.23%	0.01%