Asset retirement obligations (“ARO”) (Tables)

6 Months Ended

Jun. 30, 2012

Asset Retirement Obligation Disclosure [Abstract]

Schedule of Change in Asset Retirement Obligations

The reconciliation of the changes in the asset retirement obligations is presented below:

	Six months ended June 30,			Year ended December 31, 2011
Beginning balance, ARO liability	$	15,171		$	14,274
Additional ARO liability incurred	582			1,110
ARO liabilities settled	(690		)	(1,315		)
Accretion expense	584			1,102
Ending balance, ARO liability	$	15,647		$	15,171