Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2011

Asset Retirement Obligations

Schedule of changes in carrying amount of asset retirement obligations

	2011			2010
Balance at January 1	$	188,757		$	181,419
Accretion expense		12,469			12,499
Revisions to estimated cash flows		5,913			777
Payments		(7,506	)		(5,938	)
Balance at December 31		199,633			188,757
Less current portion		(6,926	)		(6,587	)
Asset retirement obligation, net of current portion	$	192,707		$	182,170