PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS, Additional Disclosures 3 (Details) - USD ($) $ in Millions		12 Months Ended
	Aug. 10, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Settlement loss recognized			$ (5.8)
Plan amendments		$ (0.5)	0.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		52.6	(83.5)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment		1.1	0.0
Amounts that will be amortized from accumulated other comprehensive loss in next fiscal year
Actuarial (loss) gain		(9.6)
Prior service credit		12.4
Total amortization		2.8
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement loss recognized		0.0	0.0	$ (0.5)
Plan amendments		(0.5)	0.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		51.4	(78.0)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment		1.1	0.0
Curtailment gain recognized		11.4	0.0	0.4
Expected payments		48.5
Amounts that will be amortized from accumulated other comprehensive loss in next fiscal year
Actuarial (loss) gain		(9.0)
Prior service credit		0.0
Total amortization		(9.0)
Retiree Medical Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan amendments				(7.2)
Expected payments				4.4
Plan amendment payment				2.6
Other benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement loss recognized		0.0	(5.8)	$ (0.1)
Plan amendments		0.0	0.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)		1.2	(5.5)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment		0.0	$ 0.0
Expected payments		1.7
Amounts that will be amortized from accumulated other comprehensive loss in next fiscal year
Actuarial (loss) gain		(0.6)
Prior service credit		12.4
Total amortization		11.8
Rowan Norway Limited [Member] | Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected reduction in plan participants, percentage				50.00%
Accumulated Other Comprehensive Income (Loss) [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan amendments	$ 25.9	1.3
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	(3.4)	23.6
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(3.3)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment		(9.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Increase (Decrease) for Plan Amendment	19.2	15.9
Other Noncurrent Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan amendments	14.0	0.3
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	(1.8)	6.3
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(1.9)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment		(1.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Increase (Decrease) for Plan Amendment	10.3	5.6
Other Current Liabilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan amendments	(39.9)	(1.6)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement	5.2	(29.9)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	5.2
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment		0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Increase (Decrease) for Plan Amendment	$ (29.5)	(31.5)
Income Taxes [Member]
Defined Benefit Plan Disclosure [Line Items]
Provision (benefit) for income tax in Defined Benefit Plan Curtailment		1.4
Income Taxes [Member] | Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Provision (benefit) for income tax in Defined Benefit Plan Curtailment		1.4
Other, Net [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment gain recognized		11.4
Other, Net [Member] | Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Curtailment gain recognized		$ 11.4