Asset Retirement Obligations (Tables)

6 Months Ended

Jun. 30, 2013

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Six Months Ended
	June 30,
(in thousands)	2013			2012
Beginning balance at January 1	$	86,746		$	64,019
Liabilities incurred	4,700			4,024
Liabilities settled	(2,007		)	(1,668		)
Liabilities assumed	—			1,626
Disposition of assets	(40		)	(705		)
Accretion expense	3,548			2,596
Revisions in estimated cash flows	5,907			10,988
Ending balance at June 30	$	98,854		$	80,880