Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2012

Asset Retirement Obligations

Schedule of changes in the asset retirement obligations

	December 31,
	2012			2011
Balance at beginning of year	$	74,963		$	25,257
Accretion expense		4,411			3,628
Revisions in estimated cash flows		14,353			3,722
Asset retirement obligation assumed in Western Coal acquisition(1)		—			42,599
Obligations settled		(4,249	)		(243	)
Balance at end of year	$	89,478		$	74,963

(1)

See Note 3.