Supplementary Oil and Natural Gas Disclosures (Unaudited) Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Natural Gas Reserves) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Supplementary Oil and Natural Gas Disclosures (Unaudited) [Abstract]
Future cash inflows	$ 5,401,290,000	$ 3,133,430,000
Future production costs	(1,823,303,000)	(1,141,297,000)
Future development costs	(936,580,000)	(437,074,000)
Future income taxes	(537,546,000)	(322,454,000)
Future net cash flows after income taxes	2,103,861,000	1,232,605,000
10% annual discount for estimated timing of cash flows	(529,579,000)	(356,436,000)
Standardized measure of discounted future net cash flows	1,574,282,000	876,169,000
Future Abandonment and Decommissioning Costs	466,900,000	203,700,000
Future Salvage Values	$ 32,600,000	$ 30,600,000