Asset Retirement Obligation (Tables)

9 Months Ended

Sep. 30, 2012

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Nine Months Ended
	September 30,
(in thousands)	2012			2011
Beginning balance at January 1	$	64,019		$	53,443
Liabilities incurred	5,862			2,169
Liabilities settled	(2,763		)	(1,921		)
Liabilities assumed	2,651			119
Disposition of assets	(705		)	—
Accretion expense	4,201			3,593
Revisions in estimated cash flows	10,992			4,308
Ending balance at September 30	$	84,257		$	61,711