Supplemental Gas and Oil Information (Unaudited) Capitalized costs (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Capitalized Costs, Oil and Gas Producing Activities, Net [Abstract]
Proved properties	$ 9,941.2	$ 9,574.9
Unproved properties, net	454.4	599.1
Total proved and unproved properties	10,395.6	10,174.0
Accumulated depreciation, depletion and amortization	(5,728.0)	(5,250.5)
Net capitalized costs	$ 4,667.6	$ 4,923.5

Amount of accumulated depreciation, depletion, amortization and valuation allowance relating to oil and gas producing activities.

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-Publisher FASB
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-Section 50
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The sum of capitalized costs relating to oil and gas producing activities before accounting for accumulated depreciation.

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Net amount of capitalized costs relating to oil and gas producing activities.

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Capitalized costs of proved properties incurred for any combination mineral interests acquisitions; wells and related equipment; support equipment and facilities; and uncompleted wells and equipment and other costs not previously disclosed within this table.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Capitalized costs of unproved properties incurred for any combination mineral interests acquisitions and other costs not previously disclosed within this table.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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