Asset Retirement Obligations (Tables)

6 Months Ended

12 Months Ended

Jun. 30, 2013

Dec. 31, 2012

Asset Retirement Obligations

Schedule of reconciliation of asset retirement obligations

Asset retirement obligations—beginning of period	$	10,552
Obligations incurred		44
Accretion expense		531
Asset retirement obligations—end of period	$	11,127

	2011		2012
Asset retirement obligations—beginning of year	$	5,374		6,715
Obligations incurred for wells drilled or on properties acquired		906		9,440
Obligations related to assets sold		—		(6,107	)
Accretion expense		435		504
Asset retirement obligations—end of year	$	6,715		10,552