Acquisitions and Divestitures (Details) (USD $)	Sep. 30, 2011	Dec. 31, 2010	Aug. 01, 2011 Riley Ridge Phase 2 [Member]
Riley Ridge Purchase Price Allocation
Cash payment			$ 199,554,000
Deferred payment	15,000,000	0	15,000,000
Total consideration			214,554,000
Oil and natural gas properties - proved			48,731,000
Oil and natural gas properties - unproved			12,542,000
CO2 and other non-hydrocarbon gases properties			9,741,000
Pipelines and plants			91,594,000
Other assets			48,660,000
Asset retirement obligations			(389,000)
Business Acquisition Purchase Price Allocation Assets Acquired Liabilities Assumed Excluding Contingent Consideration			210,879,000
Goodwill			3,675,000
Helium extraction rights			$ 36,700,000

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The amount of acquisition cost of a business combination allocated to proved natural resources other than mineral rights in the mining industry.

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The amount of acquisition cost of a business combination allocated to unproved natural resources other than mineral rights in the mining industry.

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Amount of potential cash payments that could result from the contingent consideration arrangement.

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Amount of cash paid to acquire the entity.

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The total cost of the acquired entity including the cash paid to shareholders of acquired entities, fair value of debt and equity securities issued to shareholders of acquired entities, the fair value of the liabilities assumed, and direct costs of the acquisition.

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The amount of acquisition cost of a business combination allocated to an identifiable intangible asset that will be amortized.

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Amount of goodwill arising from a business combination, which is the excess of the cost of the acquired entity over the amounts assigned to assets acquired and liabilities assumed.

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The amount of acquisition cost of a business combination allocated to mineral rights for acquired entities in the mining industry.

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The amount of acquisition cost of a business combination allocated to other noncurrent liabilities of the acquired entity.

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The amount of acquisition cost of a business combination allocated to a plant used in the normal course of business, not including a plant that is held-for-sale.

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