Asset Retirement Obligations (Tables)

3 Months Ended

Mar. 31, 2012

Asset Retirement Obligations [Abstract]

Changes in Asset Retirement Obligations

	Three Months Ended March  31, 2012
	(in millions)
Asset retirement obligations as of the beginning of the period, including current portion	$	425.8
Liabilities settled		(3.0	)
Divestment of properties		(0.6	)
Accretion expense		8.3
Asset retirement obligations as of the end of the period, including current portion	$	430.5