Contract Assets and Contract Liabilities (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Sep. 15, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Deferred Costs [Abstract]
Contract assets						$ 7.1	$ 2.8
Contract Liabilities [Abstract]
Contract liabilities, current						17.8	1.1
Contract liabilities		$ 38.0		$ 1.6		38.0	1.6
Movement in Deferred costs [Rollforward]
Beginning balance				2.8
Plus: contractual additions				4.8
Less: amortization				5.8
Ending balance		1.8		1.8
Demobilization Revenue [Roll Forward]
Beginning balance				0.0
Plus: contractual additions		5.3
Ending balance		5.3		5.3
Movement in Deferred Revenue [Roll Forward]
Beginning balance				1.6
Plus: contractual additions (1)				45.5
Less: amortization				9.1
Ending balance		$ 38.0		38.0
Capex [Member]
Movement in Deferred Revenue [Roll Forward]
Plus: contractual additions (1)				28.6
Deferred Revenue [Member]
Contract Liabilities [Abstract]
Contract liabilities, current						17.8	1.1
Prepaid Expenses and Other Current Assets [Member]
Deferred Costs [Abstract]
Contract assets, current						6.7	2.8
Other Noncurrent Liabilities [Member]
Contract Liabilities [Abstract]
Contract liabilities, noncurrent						20.2	0.5
Other Assets [Member]
Deferred Costs [Abstract]
Contract assets, non-current						$ 0.4	$ 0.0
Accounts Receivable [Member]
Movement in Deferred Revenue [Roll Forward]
Plus: contractual additions (1)				$ 28.6
RowanReliance [Member] | Cobalt International [Member]
Movement in Deferred Revenue [Roll Forward]
Less: amortization			$ 28.9		$ 57.5
RowanReliance [Member] | Cobalt International [Member] | Deferred Revenue [Member]
Contract Liabilities [Abstract]
Contract liabilities, current	$ 95.9
Contracts Revenue, Early Termination Of Contract	$ 95.9