Significant Financing Transactions - (Commercial Paper, Bank Loans and Letters of Credit Outstanding) (Details) (USD $)	Mar. 31, 2012
Facility Limit	$ 3,500,000,000
Outstanding Commercial Paper	1,057,000,000
Outstanding Letters of Credit	36,000,000
Facility Capacity Available	2,407,000,000
Three Year Joint Revolving Credit Facility [Member}
Facility Limit	500,000,000	[1]
Outstanding Commercial Paper	0	[1]
Outstanding Letters of Credit	36,000,000	[1]
Facility Capacity Available	464,000,000	[1]
Credit Facility 1.5 Billion | Three Year Joint Revolving Credit Facility [Member}
Facility Limit	3,000,000,000	[2]
Outstanding Commercial Paper	1,057,000,000	[2]
Outstanding Letters of Credit	0	[2]
Facility Capacity Available	1,943,000,000	[2]
Credit Facility, to support letters of credit	$ 1,500,000,000

[1] This credit facility has a maturity date of September 2016, and can be used to support bank borrowings, commercial paper and letter of credit issuances.

[2] This credit facility has a maturity date of September 2016, and can be used to support bank borrowings and the issuance of commercial paper, as well as to support up to $1.5 billion of letters of credit.