Short-Term Borrowings (Primary Components Of Short-term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Short-term Debt [Line Items]
Weighted-average interest rate at year-end	0.55%	0.64%
Securities Sold Under Repurchase Agreements
Short-term Debt [Line Items]
Balance	$ 8,006	$ 8,572	$ 7,599
Maximum outstanding at any month-end	9,306	9,853	9,058
Average outstanding during the year	7,697	9,040	8,108
Weighted-average interest rate at year-end	0.06%	0.04%	0.04%
Weighted-average interest rate during the year	0.01%	0.11%	0.05%
Federal Funds Purchased
Short-term Debt [Line Items]
Balance	399	656	7,748
Maximum outstanding at any month-end	1,145	8,259	7,748
Average outstanding during the year	784	845	1,759
Weighted-average interest rate at year-end	0.13%	0.05%	0.01%
Weighted-average interest rate during the year	0.09%	0.05%	0.05%
Tax-Exempt Investment Program
Short-term Debt [Line Items]
Balance	2,148	2,294	2,484
Maximum outstanding at any month-end	2,274	2,473	2,690
Average outstanding during the year	2,214	2,404	2,594
Weighted-average interest rate at year-end	0.17%	0.18%	0.37%
Weighted-average interest rate during the year	0.21%	0.26%	0.33%
Corporate Commercial Paper Program
Short-term Debt [Line Items]
Balance	2,318	2,384	2,799
Maximum outstanding at any month-end	2,503	2,825	2,831
Average outstanding during the year	2,382	2,449	2,791
Weighted-average interest rate at year-end	0.22%	0.22%	0.31%
Weighted-average interest rate during the year	0.23%	0.23%	0.31%
Conduit Commercial Paper Program
Short-term Debt [Line Items]
Balance		0	1,919
Maximum outstanding at any month-end		271	7,275
Average outstanding during the year		$ 113	$ 6,339
Weighted-average interest rate at year-end		0.00%	0.57%
Weighted-average interest rate during the year		0.47%	0.32%