Risk Management and Trading Activities - Net Transfers into and out of Each Level of Fair Value Hierarchy (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
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurements net transfers in	$ 18	$ 85	$ (1)	$ 244
Fair value measurements net transfers out	15	(38)	77	210
Fair value measurements net transfers into/out of	33	47	76	454
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurements net transfers in	(11)	48	(103)	(245)
Fair value measurements net transfers out	(2)	(131)	16	(288)
Fair value measurements net transfers into/out of	(13)	(83)	(87)	(533)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurements net transfers in	(14)	50	(12)	100
Fair value measurements net transfers out	(6)	(14)	23	(21)
Fair value measurements net transfers into/out of	$ (20)	$ 36	$ 11	$ 79