Fair Value Measurements Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	$ 9		$ 15
Assets, Fair Value Disclosure	82		48
Commodity derivative liabilities	(75)		(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	73		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	32		114
Assets, Fair Value Disclosure	32		114
Commodity derivative liabilities	(153)		(379)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	0		1
Assets, Fair Value Disclosure	0		1
Commodity derivative liabilities	0		0
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	(23)	[1]	(100)	[1]
Assets, Fair Value Disclosure	(23)	[1]	(100)	[1]
Commodity derivative liabilities	78	[1]	223	[1]
Net cash collateral receivable	55		123
Money market mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	73	[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	73	[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 $55 million and $123 million as of December 31, 2012 and 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.