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	Sep. 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	$ 612	$ 621
Total liabilities measured at fair value on a recurring basis	634	609
Level 2 [Member] | Commodity and Interest rate and other [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	63
Level 2 [Member] | Commodity and other [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total liabilities measured at fair value on a recurring basis	$ 1