Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2011

Asset Retirement Obligations [Abstract]

Changes in Asset Retirement Obligations

Changes in asset retirement obligations were as follows:

	Year Ended December 31,
	2011			2010
(millions)
Asset Retirement Obligations, Beginning Balance	$	253		$	232
Liabilities Incurred		23			17
Liabilities Settled		(24	)		(56	)
Revision of Estimate		105			43
Accretion Expense		20			17
Asset Retirement Obligations, Ending Balance	$	377		$	253