Asset Retirement Obligations (Tables)

3 Months Ended

Mar. 31, 2012

Asset Retirement Obligations [Abstract]

Changes in Asset Retirement Obligations

Changes in asset retirement obligations were as follows:

	Three Months Ended March 31,
	2012			2011
(millions)
Asset Retirement Obligations, Beginning Balance	$	377		$	253
Liabilities Incurred		6			1
Liabilities Settled		(2	)		(9	)
Revision of Estimate		3			4
Accretion Expense		7			5
Asset Retirement Obligations, Ending Balance	$	391		$	254