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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Balance at beginning of period	$ 107	$ 87	$ 141	$ (143)
Unrealized pre-tax gains (losses), Included in earnings	(61)	(26)	(122)	(72)
Unrealized pre-tax gains (losses), Included in other comprehensive income (loss)				43
Purchases	4	14	44	244
Sales	(1)	(9)	(32)	(266)
Settlements	(7)	15	(20)	232
Transfers into Level 3	(14)	50	(12)	100
Transfers out of Level 3	(6)	(14)	23	(21)
Balance at end of period	$ 22	$ 117	$ 22	$ 117