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Description Of The Business And Summary Of Significant Accounting Policies (Narrative) (Details)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Apr. 11, 2019
USD ($)
rigs
$ / shares
shares
Oct. 06, 2017
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
drillship
jackup
rigs
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2019
$ / shares
Dec. 31, 2019
USD ($)
Dec. 31, 2019
jackup
Dec. 31, 2019
continent
Dec. 31, 2019
rigs
Dec. 31, 2019
country
Apr. 10, 2019
$ / shares
Jan. 01, 2019
USD ($)
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     74                    
Number of jackups leased by ARO     9               9         9          
Number of different countries having drilling contracts spanning | country                                     24    
Number of different continents having drilling contracts | continent                                 6        
Number of Rigs Owned by ARO                               7   7      
Net foreign currency exchange gains (losses) | $                     $ (7.4) $ (17.2) $ (5.1)                
Short-term Investments | $             $ 329.0         329.0     $ 0.0            
Asset Impairment Charges | $     $ 13.3 $ 88.2 $ 2.5 $ 0.0 40.3 $ 0.0 $ 0.0 $ 0.0 $ 104.0 $ 40.3 $ 182.9                
Antidilutive share options excluded from computation of diluted earnings per share | shares                     300,000 1,500,000 2,000,000.0                
Operating Lease, Liability | $                             72.9            
Operating Lease, Right-of-Use Asset | $             $ 0.0         $ 0.0     $ 58.1            
Three Percent Senior Notes Due Two Thousand Twenty Four [Member] | Senior Notes [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Debt instrument, interest rate, stated percentage     3.00%               3.00%                    
Common Class A [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Common Stock, Par or Stated Value Per Share | $ / shares             $ 0.40         $ 0.40   $ 0.40              
Rowan Companies [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned, Per Share | $ / shares $ 43.67                                        
Consideration transferred | $ $ 1,400.0                   $ 1,402.8                    
Rowan Companies [Member] | Common Class A [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Entity shares issued per acquiree share | shares 2.750                                        
Shares Issued, Price Per Share | $ / shares $ 15.88                                        
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned, Per Share | $ / shares $ 2.75                                        
Number of ordinary shares | shares 88,300,000                                        
Common Stock, Par or Stated Value Per Share | $ / shares $ 0.40                                     $ 0.10  
Ultra Deepwater Drillships [Member] | Rowan Companies [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs 4                                        
Jackup Rigs [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     50                    
Jackup Rigs [Member] | Rowan Companies [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs 19                   19                    
Held-for-sale                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     1                    
Held-for-sale | Rowan Companies [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs 2                                        
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                    
Drilling rigs and equipment [Member] | Rowan Companies [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs | drillship                     4                    
Drilling rigs and equipment [Member] | Rowan Companies [Member] | Minimum                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Property, Plant and Equipment, Useful Life   30 years                 16 years                    
Drilling rigs and equipment [Member] | Rowan Companies [Member] | Maximum                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Property, Plant and Equipment, Useful Life   35 years                 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                    
Accounting Standards Update 2016-02 [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Operating Lease, Liability | $                                         $ 64.6
Operating Lease, Right-of-Use Asset | $                                         $ 53.7
Floaters [Member] | Asset under Construction [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     2                    
Floaters [Member] | Ultra Deepwater Drillships [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     16                    
Floaters [Member] | Dynamically Positioned Semisubmersible [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     8                    
Floaters [Member] | Moored Semisubmersible Rigs [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                     2