Risk Management and Trading Activities - Fair Values for Corporation's Net Physical Derivative Financial Assets and Liabilities (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	$ 854	$ 621
Total liabilities measured at fair value on a recurring basis	595	609
Commodity and Interest Rate And Other [Member] | Corporate risk management activities [Member] | Level 2 assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	193
Other [Member] | Corporate risk management activities [Member] | Level 2 liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total liabilities measured at fair value on a recurring basis	11
Commodity [Member] | Corporate risk management activities [Member] | Level 1 assets [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	$ 1