Short-Term Borrowings - Outstanding and weighted-average interest rates of short-term borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Weighted-average interest rate as of year-end	0.13%	0.05%
Securities Sold Under Repurchase Agreements
Short-term Debt [Line Items]
Balance as of December 31	$ 4,400	$ 4,499	$ 8,925
Maximum outstanding as of any month-end	5,572	10,977	10,955
Average outstanding during the year	$ 4,113	$ 8,875	$ 8,817
Weighted-average interest rate as of year-end	0.04%	0.02%	0.005%
Weighted-average interest rate for the year	0.02%	0.01%	0.00%
Federal Funds Purchased
Short-term Debt [Line Items]
Balance as of December 31	$ 0	$ 6	$ 21
Maximum outstanding as of any month-end	29	29	29
Average outstanding during the year	$ 31	$ 21	$ 20
Weighted-average interest rate as of year-end	0.00%	0.03%	0.01%
Weighted-average interest rate for the year	0.17%	0.01%	0.00%
Tax-Exempt Investment Program
Short-term Debt [Line Items]
Balance as of December 31	$ 1,158	$ 1,748	$ 1,870
Maximum outstanding as of any month-end	1,726	1,865	1,938
Average outstanding during the year	$ 1,512	$ 1,807	$ 1,903
Weighted-average interest rate as of year-end	0.67%	0.03%	0.06%
Weighted-average interest rate for the year	0.36%	0.06%	0.08%
Corporate Commercial Paper Program
Short-term Debt [Line Items]
Balance as of December 31		$ 0	$ 2,485
Maximum outstanding as of any month-end		2,919	2,485
Average outstanding during the year		$ 1,897	$ 2,136
Weighted-average interest rate as of year-end		0.00%	0.16%
Weighted-average interest rate for the year		0.26%	0.17%