Natural Gas and Oil Properties (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Discount rate of natural gas and oil reserves	10.00%
Period of time needed to calculate ceiling value of reserves	12 months
Cash flow hedges impact on ceiling value	$ 185.8
West Texas Intermediate Oil [Member]
Full cost ceiling test, Price	89.17
Henry Hub Natural Gas [Member]
Full cost ceiling test, Price	2.95

Price used in the full cost ceiling test. Prior to the fourth quarter of 2009, this price was the quoted market price as of the balance sheet date (if the quoted price at the balance sheet date resulted in an impairment, management could previously use a more favorable quoted market price at a day subsequent to the balance sheet date but prior to the auditor's report on the affected financial statements). For all periods including and subsequent to the fourth quarter of 2009, this price is an average of the prices for the first-day-of-the-month during the previous 12 months.

No definition available.

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