Environmental Matters (Accrued Liabilities And Regulatory Assets) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Site Contingency [Line Items]
Regulatory assets	$ 9,205	$ 9,501
Manufactured Gas Plant Sites [Member]
Site Contingency [Line Items]
Accrued Liabilities:	469	422
Other Superfund Sites [Member]
Site Contingency [Line Items]
Accrued Liabilities:	70	67
Superfund Sites [Member]
Site Contingency [Line Items]
Accrued Liabilities:	539	489
Regulatory assets	$ 730	$ 681