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Property And Equipment (Narrative) (Details) $ in Millions	3 Months Ended							4 Months Ended	8 Months Ended	12 Months Ended
Property And Equipment (Narrative) (Details) $ in Millions	Jun. 30, 2021 $ / bbl	Dec. 31, 2020 USD ($) $ / bbl	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) drillship	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Apr. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2021 USD ($)
Property, Plant and Equipment [Line Items]
Property and equipment		$ 13,209.3							$ 957.0	$ 13,209.3
Asset Impairment Charges								$ 756.5	0.0	3,646.2	$ 104.0
Impairment of Long-Lived Assets to be Disposed of		0.0	$ 0.0	$ 15.0		$ 10.2	$ 88.2
Impairment of Long-Lived Assets Held-for-use											0.0
Assets Held-for-sale, Not Part of Disposal Group		$ 2.3								2.3
Oil and Gas, Average Sale Price \| $ / bbl	20	60
Number of rigs impaired \| drillship				2
Gain (Loss) on Disposition of Other Assets								5.3	20.7
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment		$ 446.1							$ 35.6	446.1
Construction in Progress [Member] \| Ultra Deepwater Drillships [Member]
Property, Plant and Equipment [Line Items]
Property and equipment		$ 418.0								418.0
Other Rigs [Member]
Property, Plant and Equipment [Line Items]
Asset Impairment Charges								$ 756.5		$ 3,600.0	$ 98.4
Floaters, Jackups And Spare Equipment [Member]
Property, Plant and Equipment [Line Items]
Impairment of Long-Lived Assets Held-for-use				$ 817.3	$ 2,800.0
Property, Plant, and Equipment, Fair Value Disclosure				$ 69.0	$ 72.3
Floaters [Member]
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure												$ 26.0