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Income Taxes (Narrative) (Details) € in Millions, $ in Millions		3 Months Ended			6 Months Ended			12 Months Ended
	Apr. 11, 2019 EUR (€)	Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 EUR (€)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2019 USD ($)
Income Tax Contingency [Line Items]
Discrete Income Tax Expense (Benefit)		$ 47.3		$ 1.2		$ 211.7	$ 0.6
Income tax expense, adjusted for discrete items		31.5		$ 33.8		43.9	$ 64.7
Tax Assessment			$ 42.0
Deferred Tax Liabilities, Net		37.1				37.1			$ 99.0
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	159.6		€ 142.0
Deferred Other Tax Expense (Benefit)					79.0	88.8
Deferred Tax Liabilities, Net		104.5				104.5
Australian Taxation Office [Member] \| Rowan Companies [Member]
Income Tax Contingency [Line Items]
Tax Assessment						101.0
Unrecognized tax benefit, maximum exposure			$ 29.0			69.7
Tax Assessment Liability		$ 15.6				$ 15.6
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) \| €	€ 2.2
4.875% Senior Notes Due 2022 Member [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.58%				4.58%			4.58%
5.40% Senior Notes Due 2042 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.40%				5.40%			5.40%
7.38% Senior Notes Due 2025 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		7.38%				7.38%			7.38%
4.75% Senior Notes Due 2024 Member [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.75%				4.75%			4.75%
5.85% Senior Notes Due 2044 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.85%				5.85%			5.85%
Senior Notes \| 4.875% Senior Notes Due 2022 Member [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.875%				4.875%
Senior Notes \| 5.40% Senior Notes Due 2042 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.40%				5.40%
Senior Notes \| 7.38% Senior Notes Due 2025 [Member]
Income Tax Contingency [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		7.375%				7.375%
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%