Short-Term Borrowings - Outstanding and weighted-average interest rates of short-term borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Weighted-average interest rate as of year-end	1.64%	0.88%
Securities Sold Under Repurchase Agreements
Short-term Debt [Line Items]
Balance as of December 31		$ 1,102	$ 1,082	$ 2,842
Maximum outstanding as of any month-end	$ 4,125	3,441	4,302
Average outstanding during the year	$ 1,616	$ 2,048	$ 3,683
Weighted-average interest rate as of year-end	0.00%	1.38%	0.03%
Weighted-average interest rate during the year	1.90%	0.62%	0.05%
Tax-Exempt Investment Program
Short-term Debt [Line Items]
Balance as of December 31		$ 823	$ 931	1,078
Maximum outstanding as of any month-end	$ 931	1,078	1,158
Average outstanding during the year	$ 898	$ 1,023	$ 1,127
Weighted-average interest rate as of year-end	1.75%	1.74%	1.45%
Weighted-average interest rate during the year	1.51%	1.46%	0.79%
Other
Short-term Debt [Line Items]
Balance as of December 31		$ 0	$ 2,000	$ 0
Maximum outstanding as of any month-end	$ 0	$ 2,000	0
Average outstanding during the year	$ 3		$ 1
Weighted-average interest rate as of year-end	0.00%	2.68%	0.00%
Weighted-average interest rate during the year	0.01%		0.00%