Debt - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Debt maturing within twelve months, total	$ 708,000,000
Canadian credit facility	117,000,000	137,000,000
Senior notes and debentures	6,711,000,000	6,228,000,000
Interest rate of senior notes	6.375%
Tax-exempt bonds, current	141,000,000
Tax-exempt bonds subject to re-pricing within next 12 months	663,000,000
Tax-exempt bonds subject to re-pricing within next 12 months classifed as long-term	545,000,000
Revolving credit facility, 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
Senior notes issued	500,000,000
Senior notes interest rate	2.90%
Senior notes maturity date	Sep. 15, 2022
Net proceeds from senior notes issuance	495,000,000
Portion of the proceeds used to repay	400,000,000
Repayment of tax-exempt bonds	91,000,000
Canadian credit facility [Member]
Debt Instrument [Line Items]
Weighted average interest rate	2.00%	1.80%
Net Repayment of Canadian credit facility debt	27,000,000
Senior notes and debentures [Member]
Debt Instrument [Line Items]
Weighted average interest rate	5.80%	6.00%
Interest rate lower range	2.60%
Interest rate upper range	7.75%
End period of maturity for debt instrument	2039
Tax-exempt bonds [Member]
Debt Instrument [Line Items]
Weighted average interest rate	2.90%	3.10%
Interest rate lower range	0.20%
Interest rate upper range	7.40%
End period of maturity for debt instrument	2041
Issuance of tax-exempt bonds	43,000,000
Tax-exempt project bonds [Member]
Debt Instrument [Line Items]
Weighted average interest rate	1.40%	1.30%
Interest rate lower range	0.20%
Interest rate upper range	3.40%
End period of maturity for debt instrument	2029
Capital leases and other [Member]
Debt Instrument [Line Items]
Interest rate upper range	12.00%
End period of maturity for debt instrument	2055
6.375% Senior Notes Due November 2012
Debt Instrument [Line Items]
Senior notes and debentures	400,000,000
Revolving credit facility [Member]
Debt Instrument [Line Items]
Weighted average interest rate		1.50%
Revolving credit facility, aggregate capacity	2,000,000,000
End period of maturity for debt instruments	May 09, 2016
Repayment of net of borrowings under revolving credit facility	$ 150,000,000
Letter of Credit Facilities
Debt Instrument [Line Items]
End period of maturity for debt instruments	Jun. 30, 2015
Start period of maturity for debt instruments	Jun. 30, 2013