Fair Value Measurements Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	$ 17		$ 15
Assets, fair value disclosure	183		48
Commodity derivative liabilities	(80)		(156)
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	0		0
Assets, fair value disclosure	166		33
Commodity derivative liabilities	0		0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	52		114
Assets, fair value disclosure	52		114
Commodity derivative liabilities	(183)		(379)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	1		1
Assets, fair value disclosure	1		1
Commodity derivative liabilities	0		0
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	(36)	[1]	(100)	[1]
Assets, fair value disclosure	(36)	[1]	(100)	[1]
Commodity derivative liabilities	103	[1]	223	[1]
Cash collateral, net receivable (payable), offset against derivative positions	67		123
Money market mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	166	[2]	33	[2]
Money market mutual funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	166	[2]	33	[2]
Money market mutual funds [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	0	[2]	0	[2]
Money market mutual funds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	0	[2]	0	[2]
Money market mutual funds [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	$ 0	[1],[2]	$ 0	[1],[2]

[1] Represents netting under master netting arrangements and a net cash collateral receivable of $67 million and $123 million as of September 30, 2012 and December 31, 2011, respectively.

[2] Amounts are included in cash and cash equivalents, other current assets and other assets on the Consolidated Balance Sheets. The fair value of these money market mutual funds approximates cost.