Risk Management and Trading Activities - Changes in Physical Derivative and Financial Assets and Liabilities that are Measured at Fair Value Based on Level 3 Inputs (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Unrealized pre-tax gains (losses) in other comprehensive income	$ 43	$ 43