Acquisitions and Divestitures Terminated Williston Basin Divestiture (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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	$ 13.8	$ 678.9	$ 243.6
Impairment	$ 8.7	$ 5.0	1,560.9
ProvedAndUnprovedProperties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment			1,559.3
Williston Basin Divestiture [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Definitive Agreement Agreed Upon Sales Price			1,725.0
Cash and Contractual Rights			1,650.0
Rights To Acquire Common Stock			75.0
Impairment			$ 1,560.9

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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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-Publisher FASB
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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