Acquisitions and Divestitures Pinedale Divestiture (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	$ 678.9	$ 243.6	$ 806.8
Gain (Loss) on Disposition of Oil and Gas Property	3.9	25.0	213.5
Income (loss) before income taxes	$ (140.3)	(1,329.0)	(42.9)
Pinedale Divestiture [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			718.2
Gain (Loss) on Disposition of Oil and Gas Property		1.2	180.4
Income (loss) before income taxes			$ 251.0
Pinedale Divestiture [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase Commitment, Remaining Minimum Amount Committed		$ 9.3

Amount of gain (loss) on sale or disposal of proven and unproven oil and gas properties.

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The cash inflow to dispose of long-lived, physical assets and mineral interests in oil and gas properties used for normal oil and gas operations.

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Minimum amount to be expended to satisfy the terms of arrangements in which the entity has agreed to expend funds to procure goods or services, excluding long-term purchase commitments or unconditional purchase obligations.

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Income before income taxes for oil and gas producing activities.

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