Oil and Gas Operations (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Proved	$ 6,241,148	$ 4,927,576
Unproved	197,979	238,792
Total capitalized costs	6,439,127	5,166,368
Accumulated depreciation, depletion and amortization	1,765,241	1,382,526
Capitalized costs, net	$ 4,673,886	$ 3,783,842