Fair Value Measurements Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	$ 151		$ 82
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	142		73
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	17		32
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	0		0
Fair Value, Measurements, Recurring [Member] | Other [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	(8)	[1]	(23)	[1]
Cash collateral, net receivable, offset against derivative positions	12		55
Money market mutual funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	142	[2]	73	[2]
Money market mutual funds [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Money market mutual funds	142	[2]	73	[2]
Money market mutual funds [Member] | Fair Value, Measurements, Recurring [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] | Fair Value, Measurements, Recurring [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] | Fair Value, Measurements, Recurring [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]
Commodity Contract [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral, net receivable, offset against derivative positions	12		55
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	9		9
Commodity derivative liabilities	(54)		(75)
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	0		0
Commodity derivative liabilities	0		0
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	17		32
Commodity derivative liabilities	(74)		(153)
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	0		0
Commodity derivative liabilities	0		0
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Other [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative assets	(8)	[1]	(23)	[1]
Commodity derivative liabilities	$ 20	[1]	$ 78	[1]

[1] Represents netting under master netting arrangements and a net cash collateral receivable of $12 million and $55 million as of September 30, 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.