Financial 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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurements net transfers in	$ 40	$ 18	$ 30	$ (1)
Fair value measurements net transfers out	2	15	1	77
Fair Value Measurements Net Transfers Into Or Out Of, Total	42	33	31	76
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurements net transfers in	(6)	(11)	(22)	(103)
Fair value measurements net transfers out	(106)	(2)	(102)	16
Fair Value Measurements Net Transfers Into Or Out Of, Total	(112)	(13)	(124)	(87)
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurements net transfers in	67	(14)	74	(12)
Fair value measurements net transfers out	3	(6)	19	23
Fair Value Measurements Net Transfers Into Or Out Of, Total	$ 70	$ (20)	$ 93	$ 11