Short-Term Borrowings (Selected Data) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short Term Borrowings Summary
Maximum outstanding at any month-end during the year	$ 5,056	$ 5,196	$ 4,385
Balance outstanding at end of year	3,717	4,138	2,864
Average outstanding during the year	$ 3,421	$ 4,459	$ 3,408
Average interest rate during the year (includes derivative impact)	0.11%	0.13%	0.20%
Average interest rate at end of year	0.09%	0.12%	0.22%