Short-term Borrowings Short-term Borrowings (Summary 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]
Amount outstanding at year-end	$ 110	$ 70
Federal Funds Purchased and Securities Sold Under Agreements to Repurchase
Short-term Debt [Line Items]
Amount outstanding at year-end	87	70	126
Weighted average interest rate at year-end	0.11%	0.05%	0.12%
Maximum month-end balance during the year	87	317	474
Average balance outstanding during the year	76	137	210
Weighted average interest rate during the year	0.12%	0.09%	0.11%
Other Short-term Borrowings
Short-term Debt [Line Items]
Amount outstanding at year-end	23	0	4
Weighted average interest rate at year-end	0.00%	0.00%	4.95%
Maximum month-end balance during the year	23	18	16
Average balance outstanding during the year	$ 0	$ 1	$ 6
Weighted average interest rate during the year	0.00%	4.33%	5.31%