Fair Value (Tables)

12 Months Ended

Dec. 31, 2012

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis

	Asset fair value measurements using
	Total		Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)		Significant unobservable inputs (Level 3)
As of December 31, 2012
Energy commodity derivative contracts(a)	$	86	$	3	$	76	$	7
Interest rate swap agreements	$	603	$	—	$	603	$	—
As of December 31, 2011
Energy commodity derivative contracts(a)	$	108	$	34	$	47	$	27
Interest rate swap agreements	$	596	$	—	$	596	$	—

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	Liability fair value measurements using
	Total			Quoted prices in active markets for identical liabilities (Level 1)			Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
As of December 31, 2012
Energy commodity derivative contracts(a)	$	(33	)	$	(3	)	$	(26	)	$	(4	)
Interest rate swap agreements	$	(1	)	$	—		$	(1	)	$	—
As of December 31, 2011
Energy commodity derivative contracts(a)	$	(160	)	$	(15	)	$	(125	)	$	(20	)
Interest rate swap agreements	$	—		$	—		$	—		$	—

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(a) Level 1 consists primarily of the New York Mercantile Exchange (NYMEX) natural gas futures. Level 2 consists primarily of over-the-counter (OTC) West Texas Intermediate swaps and OTC natural gas swaps that are settled on NYMEX. Level 3 consists primarily of West Texas Intermediate options.

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation

The table below provides a summary of changes in the fair value of our Level 3 energy commodity derivative contracts for each of the years ended December 31, 2012 and 2011 (in millions):

Significant unobservable inputs (Level 3)
	Year Ended December 31,
	2012			2011
Derivatives-net asset (liability)
Beginning of Period	$	7		$	19
Total gains or (losses):
Included in earnings	(1		)	(2		)
Included in other comprehensive income	(1		)	(12		)
Purchases	3			5
Settlements	(5		)	(3		)
End of Period	$	3		$	7
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or (losses) relating to assets held at the reporting date	$	—		$	(2	)

Fair Value, by Balance Sheet Grouping

	December 31, 2012				December 31, 2011
	Carrying Value		Estimated Fair value		Carrying Value		Estimated Fair value
Total debt	$	17,330	$	18,911	$	13,876	$	14,238