Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2013

Asset Retirement Obligation Disclosure [Abstract]

Schedule of Change in Asset Retirement Obligation [Table Text Block]

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

	Years Ended December 31,
	2013			2012
	($ in millions)
Asset retirement obligations, beginning of period	$	375		$	323
Additions	20			29
Revisions(a)	8			42
Settlements and disposals	(20		)	(41		)
Accretion expense	22			22
Asset retirement obligations, end of period	$	405		$	375

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(a) Revisions in estimated liabilities during the period relate primarily to changes in estimates of asset retirement costs and include, but are not limited to, revisions of estimated inflation rates, changes in property lives, and the expected timing of settlement.