Asset Retirement Obligation (Tables)

3 Months Ended

Mar. 31, 2012

Asset Retirement Obligation

Schedule of activity in asset retirement obligation

	Three Months Ended March 31, 2012			Year Ended December 31, 2011
Balance at beginning of period	$	15,541		$	12,471
Obligations settled		(124	)		(1,030	)
Accretion expense		251			955
Revisions in estimated cash flows		1,919			2,508
Gain on settlement		—			637
Balance at end of period	$	17,587		$	15,541