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Shareholders Equity Shareholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 8,088.8				$ 8,088.8
Net loss	$ (197.5)			$ (142.9)			21.8	$ (433.5)
Shares issued under share-based compensation plans, net					3,400,000
Distributions to noncontrolling interests				(2.0)	$ (0.7)
Net other comprehensive income	(1.0)			$ (1.3)			0.7	$ (10.2)
Ending balance	$ 9,530.5						$ 9,530.5
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	205,800,000	115,200,000	115,200,000	115,200,000	111,800,000	111,800,000	115,200,000	111,800,000
Beginning balance	$ 82.5	$ 46.2	$ 46.2	$ 46.2	$ 44.8	$ 44.8	$ 46.2	$ 44.8
Equity issuance in connection with the Rowan Transaction		$ 35.2
Equity issuance in connection with the Rowan Transaction (in shares)		88,000,000.0
Shares issued under share-based compensation plans, net		2,600,000
Shares issued under share-based compensation plans, net		$ 1.1			$ 1.4
Ending balance (in shares)	205,800,000	205,800,000	115,200,000	115,200,000	115,200,000	111,800,000	205,800,000	115,200,000
Ending balance	$ 82.5	$ 82.5	$ 46.2	$ 46.2	$ 46.2	$ 44.8	$ 82.5	$ 46.2
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	8,608.4	7,230.2	7,225.0	7,209.5	7,202.4	7,195.0	7,225.0	7,195.0
Equity issuance in connection with the Rowan Transaction	(0.6)	1,365.5
Shares issued under share-based compensation plans, net		(1.1)	(0.1)		(0.4)	(0.1)
Repurchase of shares				0.1
Share-based compensation cost	9.7	13.8	5.3	7.2	7.5	7.5
Ending balance	8,617.5	8,608.4	7,230.2	7,216.6	7,209.5	7,202.4	8,617.5	7,216.6
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,084.8	679.3	874.2	1,232.0	1,387.4	1,532.7	874.2	1,532.7
Net loss	(197.1)	405.5	(190.4)	(145.0)	(151.0)	(140.1)
Dividends paid			(4.5)	(4.5)	(4.4)	(4.4)
Ending balance	887.7	1,084.8	679.3	1,082.5	1,232.0	1,387.4	887.7	1,082.5
Retained Earnings \| Accounting Standards Update 2018-02
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect due to ASU 2018-02									$ (0.8)
AOCI Attributable to Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	19.9	19.7	18.2	20.5	29.0	28.6	18.2	28.6
Net other comprehensive income	(1.0)	0.2	1.5	(1.3)	(8.5)	(0.4)
Ending balance	18.9	19.9	19.7	19.2	20.5	29.0	18.9	19.2
AOCI Attributable to Parent \| Accounting Standards Update 2018-02
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect due to ASU 2018-02									$ 0.8
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(75.0)	(74.9)	(72.2)	(72.0)	(70.0)	(69.0)	(72.2)	(69.0)
Equity issuance in connection with the Rowan Transaction		2.1
Shares issued under share-based compensation plans, net		(0.8)	0.1		(1.4)	0.1
Repurchase of shares	0.2	1.4	2.8	0.1	0.6	1.1
Ending balance	(75.2)	(75.0)	(74.9)	(72.1)	(72.0)	(70.0)	(75.2)	(72.1)
Noncontrolling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1.6	(0.2)	(2.6)	(2.3)	(2.5)	(2.1)	(2.6)	(2.1)
Net loss	(0.4)	1.8	2.4	2.1	0.9	(0.4)
Distributions to noncontrolling interests	(2.1)
Ending balance	$ (0.9)	$ 1.6	$ (0.2)	$ (2.2)	$ (2.3)	$ (2.5)	$ (0.9)	$ (2.2)