Asset Retirement Obligation (Tables)

6 Months Ended

Jun. 30, 2011

Asset Retirement Obligation

Summary of change in asset retirement obligation

	Six months ended June 30,
(in thousands)	2011			2010
Beginning balance at January 1	$	53,443		$	43,487
Liabilities incurred		1,642			1,860
Liabilities settled		(864	)		(1,534	)
Liabilities assumed		119			3,309
Accretion expense		2,545			2,191
Ending balance at June 30	$	56,885		$	49,313