Short-Term Borrowings - Outstanding and weighted-average interest rates of short-term borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Weighted-average interest rate as of year-end	0.05%	0.04%
Securities Sold Under Repurchase Agreements
Short-term Debt [Line Items]
Balance as of December 31	$ 4,499	$ 8,925	$ 7,953
Maximum outstanding as of any month-end	10,977	10,955	11,538
Average outstanding during the year	$ 8,875	$ 8,817	$ 8,436
Weighted-average interest rate as of year-end	0.02%	0.005%	0.003%
Weighted-average interest rate for the year	0.01%	0.00%	0.01%
Federal Funds Purchased
Short-term Debt [Line Items]
Balance as of December 31	$ 6	$ 21	$ 19
Maximum outstanding as of any month-end	29	29	570
Average outstanding during the year	$ 21	$ 20	$ 298
Weighted-average interest rate as of year-end	0.03%	0.01%	0.13%
Weighted-average interest rate for the year	0.01%	0.00%	0.00%
Tax-Exempt Investment Program
Short-term Debt [Line Items]
Balance as of December 31	$ 1,748	$ 1,870	$ 1,948
Maximum outstanding as of any month-end	1,865	1,938	2,135
Average outstanding during the year	$ 1,807	$ 1,903	$ 2,030
Weighted-average interest rate as of year-end	0.03%	0.06%	0.09%
Weighted-average interest rate for the year	0.06%	0.08%	0.13%
Corporate Commercial Paper Program
Short-term Debt [Line Items]
Balance as of December 31	$ 0	$ 2,485	$ 1,819
Maximum outstanding as of any month-end	2,919	2,485	2,535
Average outstanding during the year	$ 1,897	$ 2,136	$ 1,632
Weighted-average interest rate as of year-end	0.00%	0.16%	0.14%
Weighted-average interest rate for the year	0.26%	0.17%	0.18%