Fair Value Measurements Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	$ 7		$ 9
Assets, fair value disclosure	151		82
Commodity derivative liabilities	(56)		(75)
Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	0		0
Assets, fair value disclosure	144		73
Commodity derivative liabilities	0		0
Level 2 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	20		32
Assets, fair value disclosure	20		32
Commodity derivative liabilities	(104)		(153)
Level 3 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	2		0
Assets, fair value disclosure	2		0
Commodity derivative liabilities	0		0
Other [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	(15)	[1]	(23)	[1]
Assets, fair value disclosure	(15)	[1]	(23)	[1]
Commodity derivative liabilities	48	[1]	78	[1]
Cash collateral, net receivable, offset against derivative positions	33		55
Money market mutual funds [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	144	[2]	73	[2]
Money market mutual funds [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	144	[2]	73	[2]
Money market mutual funds [Member] | Level 2 [Member]
Fair Value, Assets (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 (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 (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 $33 million and $55 million as of March 31, 2013 and December 31, 2012, 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.