Supplementary Oil and Natural Gas Disclosures (Unaudited) (Costs Incurred Associated with Finding, Acquiring and Developing Proved Oil and Natural Gas Reserves) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplementary Oil and Natural Gas Disclosures (Unaudited) [Abstract]
Acquistions-Proved	$ 706,322,000	$ 261,812,000
Acquisition - Unproved	7,496,000	14,000	623,000
Exploration	43,338,000	17,129,000	31,463,000
Development	180,938,000	83,577,000	25,643,000
Costs incurred	938,094,000	362,532,000	57,729,000
Liabilities assumed in acquisition	132,109,000	26,435,000
Asset retirement obligations incurred	$ 1,000,000	$ 200,000	$ 100,000