Oil and Gas Production Processing Derivative Activities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Fair Value	$ (1,034)	$ (1,639)
Natural Gas Trades
Derivative [Line Items]
Volume	825,999,520,432,128	1,462,999,005,003,780
Fair Value	(326)	(326)
Natural Gas Trades | Swap [Member]
Derivative [Line Items]
Volume	204,000,209,141,760	0
Fair Value	(58)	0
Natural Gas Trades | Call Option [Member]
Derivative [Line Items]
Volume	491,999,241,175,040	1,356,999,212,138,500
Fair Value	(250)	(284)
Natural Gas Trades | Basis protection swaps
Derivative [Line Items]
Volume	119,000,658,870,272	105,999,792,865,280
Fair Value	(17)	(42)
Natural Gas Trades | Swaptions
Derivative [Line Items]
Volume	10,999,411,245,056	0
Fair Value	(1)	0
Crude Oil
Derivative [Line Items]
Fair Value	(708)	(1,313)
Volume Of Future Oil And Gas Production Hedged	115,500,000	118,200,000
Crude Oil | Swap [Member]
Derivative [Line Items]
Fair Value	97	15
Volume Of Future Oil And Gas Production Hedged	33,300,000	14,900,000
Crude Oil | Call Option [Member]
Derivative [Line Items]
Fair Value	(785)	(1,282)
Volume Of Future Oil And Gas Production Hedged	74,200,000	94,700,000
Crude Oil | Swaptions
Derivative [Line Items]
Fair Value	(20)	(53)
Volume Of Future Oil And Gas Production Hedged	8,000,000	7,800,000
Crude Oil | Fixed-Price Knockout Swaps
Derivative [Line Items]
Fair Value	$ 0	$ 7
Volume Of Future Oil And Gas Production Hedged	0	800,000