Asset Retirement Obligation (Tables)

3 Months Ended

Mar. 31, 2012

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Three Months Ended March 31,
(in thousands)	2012			2011
Beginning balance at January 1	$	64,019		$	53,443
Liabilities incurred	2,993			917
Liabilities settled	(467		)	(103		)
Disposition of assets	(705		)	—
Accretion expense	1,223			1,263
Revisions in estimated cash flows	2,001			—
Ending balance at March 31	$	69,064		$	55,520