Fair Value Measurements (Details) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure		$ 121	$ 48
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure		107	30
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure		16	25
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure		2	4
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	[1]	91	30
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	[1]	91	30
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	[1]	0	0
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	[1]	0	0
Investment funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Investment funds		16
Investment funds [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Investment funds		16
Investment funds [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Investment funds		0
Investment funds [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Investment funds		0
Commodity Contract [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral, net receivable, offset against derivative positions		58	28
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[2]	(4)	(11)
Commodity derivative assets		14	18
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[2]	62	39
Commodity derivative liabilities		(59)	(75)
Cash collateral, net receivable, offset against derivative positions		58	28
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		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]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		16	25
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		(121)	(114)
Commodity Contract [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets (Liabilities) Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement		2	4
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement		$ 0	$ 0

[1] Amounts are included in cash and cash equivalents, other current assets and other assets on the Consolidated Balance Sheets. Money market mutual funds are accounted for as available-for-sale securities and the fair value approximates cost.

[2] Represents netting under master netting arrangements and a net cash collateral receivable of $58 million and $28 million as of June 30, 2015 and December 31, 2014, respectively.