Debt (Details Textual) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Debt (Textual) [Abstract]
Senior notes and debentures, maturing through 2039, interest rates ranging from 2.60% to 7.75% (weighted average interest rate of 6.0% at March 31, 2012 and December 31, 2011)	6,223,000,000	6,228,000,000
Debt (Additional) (Textual) [Abstract]
Debt maturing within twelve months, total	1,005,000,000
Canadian credit facility principal amount outstanding net of discount	115,000,000	137,000,000
Outstanding borrowing under revolving credit facility	300,000,000	150,000,000
Tax-exempt bonds, current	134,000,000
Tax-exempt bonds subject to re-pricing within next 12 months	429,000,000
Debt maturing within 12 months classified as long-term	145,000,000
Current portion of long-term debt	860,000,000	631,000,000
Repayment of tax-exempt bonds	35,000,000
Revolving credit and letter of credit facilities, aggregate capacity	2,500,000,000
Letter of credit outstanding, revolving credit and letter of credit facilities.	1,500,000,000
Letter of credit outstanding, revolving credit facility.	1,000,000,000
6.375% senior notes due November 2012 [Member]
Debt (Textual) [Abstract]
Senior notes and debentures, maturing through 2039, interest rates ranging from 2.60% to 7.75% (weighted average interest rate of 6.0% at March 31, 2012 and December 31, 2011)	400,000,000
Capital leases and other [Member]
Debt (Textual) [Abstract]
Interest rate upper range	0.12	0.12
End period of maturity for debt instruments	2055
Senior notes and debentures [Member]
Debt (Textual) [Abstract]
Interest rate lower range	0.0260
Interest rate upper range	0.0775
Weighted average interest rate	6.00%	6.00%
End period of maturity for debt instruments	2039
Tax-exempt project bonds [Member]
Debt (Textual) [Abstract]
Interest rate lower range	0.002
Interest rate upper range	0.034
Weighted average interest rate	1.40%	1.30%
End period of maturity for debt instruments	2029
Tax-exempt bonds [Member]
Debt (Textual) [Abstract]
Interest rate lower range	0.002
Interest rate upper range	0.074
Weighted average interest rate	3.10%	3.10%
End period of maturity for debt instruments	2041
Canadian credit facility [Member]
Debt (Textual) [Abstract]
Weighted average interest rate	1.60%	1.80%
Net Repayment of Canadian credit facility debt	25,000,000
Revolving credit facility [Member]
Debt (Textual) [Abstract]
Weighted average interest rate	1.40%	1.50%
End period of maturity for debt instruments	2016
Revolving credit facility, total capacity	2,000,000,000
Borrowings under revolving credit facility	150,000,000
Letter of credit facility [Member]
Debt (Textual) [Abstract]
Start period of maturity for debt instruments	2013
End period of maturity for debt instruments	2015