Asset Retirement Obligations (Tables)

3 Months Ended

Mar. 31, 2013

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Three Months Ended
	March 31,
(in thousands)	2013			2012
Beginning balance at January 1	$	86,746		$	64,019
Liabilities incurred	2,711			2,993
Liabilities settled	(805		)	(467		)
Disposition of assets	—			(705		)
Accretion expense	1,704			1,223
Revisions in estimated cash flows	4,311			2,001
Ending balance at March 31	$	94,667		$	69,064