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Income Taxes (Narrative) (Details) € in Millions, $ in Millions		3 Months Ended				9 Months Ended
	Apr. 11, 2019 EUR (€)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 EUR (€)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Income Tax Contingency [Line Items]
Discrete Income Tax Expense (Benefit)		$ (13.8)			$ 18.4		$ 197.9	$ 19.0
Income tax expense, adjusted for discrete items		8.1			$ 19.9		52.0	$ 84.6
Tax Assessment				$ 42.0
Deferred Tax Liabilities, Net		37.3		99.0			37.3
Deferred Tax Assets, Net		880.0					880.0
Unrecognized Tax Benefits		120.0					120.0
Deferred Tax Assets, Capital Loss Carryforwards		5,600.0					5,600.0
Rowan Companies [Member]
Income Tax Contingency [Line Items]
Deferred Tax Assets, Gross		66.0					66.0
Luxembourg Inland Revenue [Member] \| Rowan Companies [Member]
Income Tax Contingency [Line Items]
Tax Assessment \| €	€ 93.0
Unrecognized tax benefit, maximum exposure			€ 142.0			€ 142.0	166.5
Deferred Other Tax Expense (Benefit)						79.0	92.6
Deferred Tax Liabilities, Net		109.0					109.0
Australian Taxation Office [Member] \| Rowan Companies [Member]
Income Tax Contingency [Line Items]
Tax Assessment							101.0
Unrecognized tax benefit, maximum exposure				$ 29.0			72.3
Tax Assessment Liability		$ 16.4					$ 16.4
Euro Member Countries, Euro \| Luxembourg Inland Revenue [Member] \| Rowan Companies [Member]
Income Tax Contingency [Line Items]
Other Tax Expense (Benefit) \| €						€ 2.0
United States of America, Dollars \| Luxembourg Inland Revenue [Member] \| Rowan Companies [Member]
Income Tax Contingency [Line Items]
Deferred Other Tax Expense (Benefit) \| €	€ 2.3
4.875% Senior Notes Due 2022 Member [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		487.50%					487.50%
5.40% Senior Notes Due 2042 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		540.00%					540.00%
7.38% Senior Notes Due 2025 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		737.50%					737.50%
4.75% Senior Notes Due 2024 Member [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		475.00%					475.00%
5.85% Senior Notes Due 2044 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		585.00%					585.00%
Senior Notes \| 4.75% Senior Notes Due 2024 Member [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.75%					4.75%
Senior Notes \| 5.85% Senior Notes Due 2044 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.85%					5.85%