Supplemental Gas and Oil Information (Unaudited) Costs incurred (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Gas and Oil Information (Unaudited) [Abstract]
Accrued capital costs included in development costs	$ 81.3
Costs Incurred, Asset Retirement Obligation Incurred	17.7
Costs Incurred To Advance The Development Of Proved Undeveloped Reserves	513.0	533.6	434.2
Unproved	692.6	48.0	109.1
Proved	714.4	0.1	0.2
Total Property Acqusitions	1,407.0	48.1	109.3
Costs Incurred, Exploration Costs	14.3	36.5	146.4
Costs Incurred, Development Costs	1,310.0	1,267.8	988.8
Costs Incurred, Acquisition of Oil and Gas Properties	$ 2,731.3	$ 1,352.4	$ 1,244.5

Represents capital costs accrued for the development of proved reserves.

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The costs incurred during the period to advance the development of reserves that were classified as proved undeveloped.

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Total Proved and Unproved Property Acqusitions Costs Incurred

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Cost incurred, including capitalized costs and costs charged to expense, in acquisition of oil and gas properties.

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have proved reserves.

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have unproved reserves.

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Amount of asset retirement obligations (ARO) costs incurred (whether capitalized or charged to expense) during the period.

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Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

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Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

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