Commercial Paper Outstanding Commercial Paper Balances Table (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commercial Paper [Abstract]
Balance	$ 0	$ 4,026	$ 3,157
Highest aggregate month-end balance	4,660	4,304	4,864
Average borrowings	$ 1,647	$ 3,815	$ 3,732
Weighted Average Interest Rate:
At year-end	0.00%	0.30%	0.30%
Paid during year	0.30%	0.20%	0.30%