Short-Term Borrowings - Outstanding and weighted-average interest rates of short-term borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Weighted-average interest rate as of year-end	0.25%	0.13%
Securities Sold Under Repurchase Agreements
Short-term Debt [Line Items]
Balance as of December 31	$ 2,842	$ 4,400	$ 4,499
Maximum outstanding as of any month-end	4,302	5,572	10,977
Average outstanding during the year	$ 3,683	$ 4,113	$ 8,875
Weighted-average interest rate as of year-end	0.03%	0.04%	0.02%
Weighted-average interest rate during the year	0.05%	0.02%	0.01%
Federal Funds Purchased
Short-term Debt [Line Items]
Balance as of December 31	$ 0	$ 0	$ 6
Maximum outstanding as of any month-end	0	29	29
Average outstanding during the year	$ 1	$ 31	$ 21
Weighted-average interest rate as of year-end	0.00%	0.00%	0.03%
Weighted-average interest rate during the year	0.00%	0.17%	0.01%
Tax-Exempt Investment Program
Short-term Debt [Line Items]
Balance as of December 31	$ 1,078	$ 1,158	$ 1,748
Maximum outstanding as of any month-end	1,158	1,726	1,865
Average outstanding during the year	$ 1,127	$ 1,512	$ 1,807
Weighted-average interest rate as of year-end	1.45%	0.67%	0.03%
Weighted-average interest rate during the year	0.79%	0.36%	0.06%