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Income Taxes (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2018
Income from continuing operations before income taxes in the U.S. countries     $ 6.3 $ (151.6) $ (578.2)  
Income from continuing operations before income taxes in the non-U.S. countries     (202.3) 1,100.0 (893.0)  
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) $ 16.5          
Tax Cuts and Jobs Act of 2017, transition tax 38.5          
Tax Cuts and Jobs Act of 2017, tax benefit on re-measurement on deferred tax assets and liabilities 17.3          
Tax Cuts and Jobs Act of 2017, tax benefit from adjustment to valuation allowance 20.0          
Tax Cuts and Jobs Act of 2017, tax benefit on valuation allowance 19.3          
Deferred tax assets related to U.S. foreign tax credits 132.3   132.3 91.7    
Deferred tax assets related to net operating loss carryforwards 187.1   187.1 197.9    
Net operating loss carryforwards 844.1   844.1      
Operating loss carryforwards, Not subject to expiration 429.2   429.2      
Operating loss carryforwards, Subject to expiration 414.9   414.9      
Valuation allowance on NOL carryforwards and FTC 250.3   250.3      
Income Tax Expense (Benefit), Discrete Item     32.2 8.4    
Other discrete income tax expenses     $ 15.7      
Valuation Allowance, Deferred Tax Asset, Change in Amount       5.5 $ 11.0  
Unrecognized Tax Benefits Associated with Liabilities for Unrecognized Tax Benefits       $ 5.3    
Consilidated effective income tax rate excluding discrete items     (96.00%) 20.30% 16.00%  
Minimum percentage threshold for recognition of tax benefits     50.00%      
Total unrecognized tax benefits 147.6   $ 147.6 $ 122.0 $ 140.6  
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions With Net Operating Loss Carryforwards     8.2 5.7    
Amount of unrecognized tax benefits affecting the consolidated effective income tax rate if recognized 130.3   130.3      
Amount of accrued interest and penalties included in other liabilities 38.6   38.6 26.6    
Amount of interest and penalties recognized in net tax expense     4.4 3.8 3.9  
Income tax benefits, inclusive of interest and penalties due to lapses in statute of limitations 1.1   1.1 0.6 7.6  
Unamortized deferred charges related to intercompany transfers       33.0    
Amount included in current income taxes for amortization of deferred income taxes related to intercompany transfers       4.1 2.6  
Deferred tax liability related to temporary difference from transferred drilling rigs       18.9    
Tax benefits included in deferred income tax expense related to amortization of deferred reversing temporary differences from intercompany transfers       2.3 1.8  
Other Liabilities [Member]            
Total unrecognized tax benefits 139.4   $ 139.4 116.3    
Rig Impairments [Member]            
Income Tax Expense (Benefit), Discrete Item         $ 192.5  
Minimum [Member]            
Deferred Tax Assets, Foreign, Expiration Date     2022      
Operating loss carryforwards tax credits expiration year     2018      
Maximum [Member]            
Deferred Tax Assets, Foreign, Expiration Date     2038      
Operating loss carryforwards tax credits expiration year     2037      
Other Operating Income (Expense) [Member]            
Income Tax Expense (Benefit), Discrete Item       $ 16.9    
Scenario, Forecast            
Decline in unrecognized tax benefits during next twelve months           $ 3.6
Accrued interest and penalty assessments related to the decline in unrecognized tax benefits           $ 1.0
Accounting Standards Update 2016-16 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]            
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification   $ 14.1        
Atwood Oceanics, Inc.            
Amount of accrued interest and penalties included in other liabilities $ 7.7   $ 7.7