Oil and Gas Properties, Buildings and Equipment (Tables)

12 Months Ended

Dec. 31, 2012

Oil and Gas Properties, Buildings and Equipment [Abstract]

Schedule of Oil and Gas Properties, Building and Equipment Activities

	Year Ended December 31,
(in thousands)	2012			2011
Oil and natural gas:
Proved properties(1)	$	4,060,106		$	3,257,321
Unproved properties(2)	236,626			228,486
	4,296,732			3,485,807
Less accumulated depreciation, depletion and amortization	(1,180,298		)	(962,154		)
	3,116,434			2,523,653
Commercial and other:
Machinery, drilling rigs and equipment	18,131			22,509
Buildings and improvements	10,110			6,894
Vehicles	9,167			7,806
	37,408			37,209
Less accumulated depreciation	(25,340		)	(29,469		)
	12,068			7,740
	$	3,128,502		$	2,531,393

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(1) Includes cogeneration facilities.

(2) Unproved properties includes acquisition costs for properties to which proved developed producing and proved undeveloped reserves are also attributed. At December 31, 2012, unproved properties included $25.2 million of acquisition costs for unevaluated properties. The Company assesses these properties annually and recorded impairments of $0.1 million, $0.6 million and $0.0 million for the years ended December 31, 2012, 2011 and 2010, respectively.