Short-Term Borrowings (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Short-term Borrowings
Balance at year end, Federal Funds Purchase	$ 25	$ 32	$ 160
Balance at year end, Securities Sold Under Repurchase Agreements	1,686	2,013	1,582
Balance at year end, Other Short-Term Borrowings	337	1,151	340
Federal Funds Purchased [Member]
Short-term Borrowings
Average during the year	120	118	143
Maximum month-end balance	844	1,050	214
Weighted-average rate during the year	0.13%	0.15%	0.16%
Weighted-average rate at December 31	0.08%	0.14%	0.11%
Securities Sold under Agreements to Repurchase [Member]
Short-term Borrowings
Average during the year	1,861	1,926	1,475
Maximum month-end balance	2,286	2,305	1,582
Weighted-average rate during the year	0.28%	0.32%	0.32%
Weighted-average rate at December 31	0.25%	0.29%	0.32%
Other Short term Borrowings [Member]
Short-term Borrowings
Average during the year	619	545	1,907
Maximum month-end balance	$ 1,007	$ 1,151	$ 5,078
Weighted-average rate during the year	1.84%	2.63%	0.84%
Weighted-average rate at December 31	1.60%	2.64%	3.22%