Asset Retirement Obligation (Tables)

6 Months Ended

Jun. 30, 2012

Asset Retirement Obligation Disclosure [Abstract]

Summary of change in asset retirement obligation

	Six Months Ended June 30,
(in thousands)	2012			2011
Beginning balance at January 1	$	64,019		$	53,443
Liabilities incurred	4,024			1,642
Liabilities settled	(1,668		)	(864		)
Liabilities assumed	1,626			119
Disposition of assets	(705		)	—
Accretion expense	2,596			2,545
Revisions in estimated cash flows	10,988			—
Ending balance at June 30	$	80,880		$	56,885