Oil and Gas Operations (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Proved	$ 4,927,576	$ 3,868,945
Unproved	238,792	211,834
Total capitalized costs	5,166,368	4,080,779
Accumulated depreciation, depletion and amortization	1,382,526	1,161,635
Capitalized costs, net	$ 3,783,842	$ 2,919,144