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, 2013	Dec. 31, 2012
Supplementary Oil and Natural Gas Disclosures (Unaudited) [Abstract]
Future cash inflows	$ 5,937,536,000	$ 5,401,290,000
Future production costs	(1,957,868,000)	(1,823,303,000)
Future development costs	(1,085,440,000)	(936,580,000)
Future income taxes	(641,536,000)	(537,546,000)
Future net cash flows after income taxes	2,252,692,000	2,103,861,000
10% annual discount for estimated timing of cash flows	(603,614,000)	(529,579,000)
Standardized measure of discounted future net cash flows	1,649,078,000	1,574,282,000
Future Abandonment and Decommissioning Costs	569,900,000	466,900,000
Future Salvage Values	$ 25,800,000	$ 32,600,000