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 (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Balance at beginning of period	$ 70	$ (268)	$ 141	$ (143)
Unrealized pre-tax gains (losses)	2	120	(61)	(3)
Purchases	12	10	40	230
Sales	(8)	(23)	(31)	(257)
Settlements	2	180	(13)	217
Transfers into Level 3		23	2	50
Transfers out of Level 3	29	45	29	(7)
Balance at end of period	$ 107	$ 87	$ 107	$ 87