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Description Of The Business And Summary Of Significant Accounting Policies (Narrative) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 11, 2019
USD ($)
$ / shares
shares
Jan. 29, 2019
shares
Oct. 07, 2018
shares
Jan. 01, 2018
USD ($)
Oct. 06, 2017
USD ($)
Oct. 05, 2017
USD ($)
shares
Dec. 31, 2018
USD ($)
continent
country
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Mar. 31, 2016
USD ($)
shares
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
continent
country
shares
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
Jan. 01, 2019
USD ($)
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 56        
Number of different countries having drilling contracts spanning | country             14                   14        
Number of different continents having drilling contracts | continent             6                   6        
Bargain purchase gain                               $ 142,000,000 $ 1,800,000   $ 140,200,000 $ 0  
Net foreign currency exchange gains (losses)                             $ (6,000,000)   (17,200,000)   (5,100,000)    
Short-term Investments             $ 329,000,000       $ 440,000,000           329,000,000   440,000,000    
Asset Impairment Charges             $ 40,300,000 $ 0 $ 0 $ 0 $ 182,900,000 $ 0 $ 0 $ 0     $ 40,300,000   $ 182,900,000 $ 0  
Antidilutive share options excluded from computation of diluted earnings per share | shares                             500,000   1,500,000   2,000,000    
Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings       $ 800,000                                  
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%        
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     2.215                                    
Atwood Oceanics, Inc.                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 4        
Consideration transferred         $ 770,700,000 $ 781,800,000                   781,800,000          
Cash settlement for business acquisitions           11,100,000                              
Cash and cash equivalents         445,400,000     $ 445,400,000               $ 445,400,000   $ 445,400,000      
Payments to extinguish acquiree liabilities           $ 1,300,000,000                              
Bargain purchase gain         $ 140,200,000                         $ 1,800,000      
Atwood Oceanics, Inc. | Common Class A [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Entity shares issued per acquiree share | shares           1.60                              
Number of ordinary shares | shares           132,200,000                              
Ultra Deepwater Drillships [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 12        
Semisubmersible Rigs [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 9        
Moored Semisubmersible Rigs [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 3        
Moored Semisubmersible Rigs [Member] | Atwood Oceanics, Inc.                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 2        
Jackup Rigs [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 35        
Jackup Rigs [Member] | Atwood Oceanics, Inc.                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 5        
Construction in Progress [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 3        
Construction in Progress [Member] | Atwood Oceanics, Inc.                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of contract drilling rigs                                 2        
Drilling rigs and equipment [Member] | Minimum [Member]                                          
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 [Member]                                          
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] | Atwood Oceanics, Inc. | Minimum [Member]                                          
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] | Atwood Oceanics, Inc. | Maximum [Member]                                          
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 [Member]                                          
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 [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Property, Plant and Equipment, Useful Life                                 30 years        
Equipment [Member] | Minimum [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Property, Plant and Equipment, Useful Life                                 3 years        
Equipment [Member] | Maximum [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Property, Plant and Equipment, Useful Life                                 6 years        
Scenario, Forecast [Member] | Rowan Companies [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Consideration transferred $ 1,500,000,000.0                                        
Business Acquisition, Share Price | $ / shares $ 4.41                                        
Scenario, Forecast [Member] | Rowan Companies [Member] | Common Class A [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Number of ordinary shares | shares 351,000,000                                        
Accounting Standards Update 2016-02 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member] | Scenario, Forecast [Member] | Minimum [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Operating Lease, Liability                                         $ 75,000,000
Operating Lease, Right-of-Use Asset                                         65,000,000
Accounting Standards Update 2016-02 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member] | Scenario, Forecast [Member] | Maximum [Member]                                          
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                                          
Operating Lease, Liability                                         80,000,000
Operating Lease, Right-of-Use Asset                                         $ 70,000,000
Subsequent Event [Member] | 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