Short-Term Borrowings (Summary of Short-Term Borrowings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Amount outstanding at year-end	$ 70	$ 130
Federal Funds Purchased and Securities Sold under Agreements to Repurchase [Member]
Amount outstanding at year-end	70	126	462
Weighted average interest rate at year-end	0.05%	0.12%	0.03%
Maximum month-end balance during the year	317	474	655
Average balance outstanding during the year	137	210	467
Weighted average interest rate during the year	0.09%	0.11%	0.19%
Other Short-Term Borrowings [Member]
Amount outstanding at year-end	0	4	0
Weighted average interest rate at year-end	0.00%	4.95%	0.00%
Maximum month-end balance during the year	18	16	2,558
Average balance outstanding during the year	$ 1	$ 6	$ 532
Weighted average interest rate during the year	4.33%	5.31%	0.28%