Risk Management and Trading Activities - Gross Volume of Corporation's Derivative Instruments Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Interest rate swaps (millions of U.S. Dollars)	$ 880	$ 895
Gross volume of the Corporate risk management derivative instruments outstanding [Member]
Derivative [Line Items]
Foreign exchange (millions of U.S. Dollars)	1,285	900
Interest rate swaps (millions of U.S. Dollars)	880	895
Gross volume of derivative contracts outstanding relating to trading activities [Member]
Derivative [Line Items]
Foreign exchange (millions of U.S. Dollars)	412	581
Interest rate swaps (millions of U.S. Dollars)	$ 167	$ 182
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	26	28
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	1,179	2,169
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	2,938	2,616
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	3,377	4,203
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	278	244
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	19	304
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	1	51
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	16