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Description Of The Business And Summary Of Significant Accounting Policies (Narrative) (Details)
shares in Millions, $ in Millions
 12 Months Ended
Aug. 18, 2020
Dec. 31, 2020
USD ($)
rigs
floater
jackup
shares
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2018
USD ($)
shares
Dec. 31, 2020
country
Dec. 31, 2020
continent
Dec. 31, 2020
Dec. 31, 2020
drillship
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Number of contract drilling rigs | rigs   61            
Number of jackups leased by ARO   9           9
Number of different countries having drilling contracts spanning | country         9      
Number of different continents having drilling contracts | continent           5    
Net foreign currency exchange gains (losses) | $   $ (11.0) $ (7.4) $ (17.2)        
Asset Impairment Charges | $   $ 3,646.2 $ 104.0 $ 40.3        
Antidilutive share options excluded from computation of diluted earnings per share | shares   0.4 0.3 1.5        
Plan of Reorganization, Support of Revolving Credit Facility, Percentage   100.00%            
Holders Of Equity Interests [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Plan Of Reorganization, Warrants For New Equity 7.00%              
Plan Of Reorganization, Warrants Term 7 years              
Senior Noteholders [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Plan Of Reorganization, Pro Rata Share Percentage 39.00%              
Plan Of Reorganization, Recovery Of Claims, Percentage 100.00%              
3.00% Exchangeable senior notes due 2024                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Debt instrument, interest rate, stated percentage             3.00%  
Drilling rigs and equipment [Member] | Minimum                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Property, Plant and Equipment, Useful Life   4 years            
Drilling rigs and equipment [Member] | Maximum                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Property, Plant and Equipment, Useful Life   35 years            
Building and Building Improvements [Member] | Minimum                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Property, Plant and Equipment, Useful Life   7 years            
Building and Building Improvements [Member] | Maximum                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Property, Plant and Equipment, Useful Life   30 years            
Equipment [Member] | Minimum                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Property, Plant and Equipment, Useful Life   3 years            
Equipment [Member] | Maximum                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Property, Plant and Equipment, Useful Life   6 years            
Other Rigs [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Asset Impairment Charges | $   $ 3,600.0 $ 98.4 $ 40.3        
Floaters [Member] | Ultra Deepwater Drillships [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Number of contract drilling rigs   11            
Floaters [Member] | Dynamically Positioned Semisubmersible [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Number of contract drilling rigs   4            
Floaters [Member] | Moored Semisubmersible Rigs [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Number of contract drilling rigs   1            
Jackups [Member]                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Number of contract drilling rigs | jackup   45            
ARO                
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                
Number of contract drilling rigs   7