Asset Retirement Obligation

12 Months Ended

Dec. 31, 2012

Asset Retirement Obligation Disclosure [Abstract]

Asset Retirement Obligation Disclosure [Text Block]

Asset Retirement Obligations

The components of the change in our asset retirement obligations are shown below.

	Years Ended December 31,
	2012			2011
	($ in millions)
Asset retirement obligations, beginning of period	$	323		$	301
Additions	29			20
Revisions(a)	42			(1		)
Settlements and disposals	(41		)	(16		)
Accretion expense	22			19
Asset retirement obligations, end of period	$	375		$	323

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(a) Revisions in estimated liabilities can result from changes in estimated service and equipment costs, changes in the estimated timing of settling asset retirement obligations and changes in estimated inflation rates. In 2012, we revised our asset retirement obligations related to natural gas and oil properties based on an increase in estimated service and equipment costs and changes to the estimated timing of settling the asset retirement obligations.