Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2012

Asset Retirement Obligations

Carrying value of asset retirement obligations

		2012		2011 (1)
Beginning balance as of January 1,		$	344,180	$	341,838
Additions			59,847		47,426
Revisions in estimated timing and cash flows, net of settlements			6,641		(70,755)
Accretion expense			25,056		25,671
Balance as of December 31,		$	435,724	$	344,180
(1)	December 31, 2011 balances have been revised to reflect purchase accounting measurement period adjustments.