Supplemental Gas and Oil Information (Unaudited) Results of Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Results of Operations, Income before Income Taxes [Abstract]
Revenues (1)	$ 2,374.6	$ 1,901.2	$ 1,393.4
Production costs	735.6	583.3	501.1
Exploration expenses	9.9	11.9	11.2
Depreciation, depletion and amortization	984.4	954.2	838.4
Abandonment and impairment	1,143.2	93.0	133.0
Total expenses	2,873.1	1,642.4	1,483.7
Income before income taxes	(498.5)	258.8	(90.3)
Income taxes	182.5	(96.3)	33.6
Results of operations from producing activities excluding allocated corporate overhead and interest expenses	$ (316.0)	$ 162.5	$ (56.7)

Depreciation, depletion and amortization, and valuation provisions (impairments) related to oil and gas producing activities.

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The amount of expenses related to oil and gas producing activities.

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Exploration costs from oil and gas producing activities.

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Costs related to the impairment of oil and gas properties.

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-Publisher FASB

-Name Accounting Standards Codification

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Income before income taxes for oil and gas producing activities.

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Income tax expense for oil and gas producing activities.

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The results of operations for oil and gas producing activities, excluding corporate overhead and interest costs.

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Production (lifting) costs from oil and gas producing activities, including but not limited to lease operating expense, production and ad valorem taxes, and transportation expense.

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Total revenues from oil and gas producing activities.

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