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Shareholders' Equity (Schedule Of Activity In Our Various Shareholders' Equity) (Details) - USD ($) shares in Millions, $ in Millions		3 Months Ended										12 Months Ended
	Oct. 05, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE					$ 8,088.8							$ 8,088.8
Net loss		$ (214.0)	$ (197.5)	$ 407.3	$ (188.0)	$ (203.1)	$ (142.9)	$ (150.1)	$ (140.5)			(192.2)	$ (636.6)	$ (304.2)
Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits												(21.7)	0.0	0.0
Net other comprehensive income (loss)												(12.0)	(11.2)	$ 9.6
BALANCE		$ 9,309.6				$ 8,088.8						$ 9,309.6	$ 8,088.8
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE, shares					115.2				111.8		77.6	115.2	111.8	77.6
BALANCE					$ 46.2				$ 44.8		$ 31.1	$ 46.2	$ 44.8	$ 31.1
Stock Issued During Period, Value, New Issues													$ 13.2
Shares Issued Under Share Based Compensation Plans, Shares										1.1		2.7	3.4
Shares issued under share-based compensation plans, net										$ 0.5		$ 1.1	$ 1.4
BALANCE, shares		205.9				115.2				111.8		205.9	115.2	111.8
BALANCE		$ 82.5				$ 46.2				$ 44.8		$ 82.5	$ 46.2	$ 44.8
Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE					7,225.0				7,195.0		6,402.2	7,225.0	7,195.0	6,402.2
Shares issued under share-based compensation plans, net										(0.4)		(1.3)	(0.6)
Share-based compensation cost											35.7	37.2	30.6
Payments of Stock Issuance Costs		(0.6)
BALANCE		8,627.8				7,225.0				7,195.0		8,627.8	7,225.0	7,195.0
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE					874.2				1,532.7		1,864.1	874.2	1,532.7	1,864.1
Net loss											(303.7)	(198.0)	(639.7)
Cash dividends paid											(13.6)	(4.5)	(18.0)
BALANCE		671.7				874.2				1,532.7		671.7	874.2	1,532.7
AOCI [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE					18.2				28.6		19.0	18.2	28.6	19.0
Increase (Decrease) in Obligation, Pension and Other Postretirement Benefits												(21.7)
Net other comprehensive income (loss)											9.6	9.7	(11.2)
BALANCE		6.2				18.2				28.6		6.2	18.2	28.6
Treasury Shares [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE					(72.2)				(69.0)		(65.8)	(72.2)	(69.0)	(65.8)
Shares issued under share-based compensation plans, net										(1.3)		(0.7)	(1.3)
Repurchase of shares											(1.9)	(4.5)	(1.9)
BALANCE		(77.3)				(72.2)				(69.0)		(77.3)	(72.2)	(69.0)
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
BALANCE					$ (2.6)				$ (2.1)		4.4	(2.6)	(2.1)	4.4
Net loss											(0.5)	5.8	3.1
Distributions to noncontrolling interests											$ (6.0)	(4.5)	(3.6)
BALANCE		$ (1.3)				$ (2.6)				$ (2.1)		(1.3)	(2.6)	(2.1)
Accounting Standards Update 2016-16 [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification														$ 14.1
New Accounting Pronouncement, Early Adoption, Effect [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification													(0.8)
New Accounting Pronouncement, Early Adoption, Effect [Member] | AOCI [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification													0.8
Atwood Oceanics, Inc. | Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues	33.1
Atwood Oceanics, Inc. | Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues													757.5
Rowan Companies [Member] | Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues				88.0
Stock Issued During Period, Value, New Issues												$ 35.2
Rowan Companies [Member] | Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues													1,367.5
Rowan Companies [Member] | Treasury Shares [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues													$ 0.1

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