Risk Management and Trading Activities - Gross Volume of Corporation's Derivative Instruments Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Interest rate swap [Member]
Derivative [Line Items]
Derivative Notional Amount	$ 865	$ 880
Gross volume of the Corporate risk management derivative instruments outstanding [Member] | Foreign exchange [Member]
Derivative [Line Items]
Derivative Notional Amount	55	1,285
Gross volume of the Corporate risk management derivative instruments outstanding [Member] | Interest rate swap [Member]
Derivative [Line Items]
Derivative Notional Amount	865	880
Gross volume of derivative contracts outstanding relating to trading activities [Member] | Foreign exchange [Member]
Derivative [Line Items]
Derivative Notional Amount	89	412
Gross volume of derivative contracts outstanding relating to trading activities [Member] | Interest rate swap [Member]
Derivative [Line Items]
Derivative Notional Amount	$ 49	$ 167
Commodity, primarily crude oil (millions of barrels) [Member] | Gross volume of the Corporate risk management derivative instruments outstanding [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	8	1
Crude oil and refined petroleum products (millions of barrels) [Member] | Gross volume of the Corporation's energy marketing commodity contracts outstanding [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	20	26
Crude oil and refined petroleum products (millions of barrels) [Member] | Gross volume of derivative contracts outstanding relating to trading activities [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	1,624	1,179
Natural gas (millions of mcf) [Member] | Gross volume of the Corporation's energy marketing commodity contracts outstanding [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	4,032	2,938
Natural gas (millions of mcf) [Member] | Gross volume of derivative contracts outstanding relating to trading activities [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	3,116	3,377
Electricity (millions of megawatt hours) [Member] | Gross volume of the Corporation's energy marketing commodity contracts outstanding [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	238	278
Electricity (millions of megawatt hours) [Member] | Gross volume of derivative contracts outstanding relating to trading activities [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	5	19
Equity securities (millions of shares) [Member] | Gross volume of derivative contracts outstanding relating to trading activities [Member]
Derivative [Line Items]
Equity securities (millions of shares)	14	14