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Description Of The Business And Summary Of Significant Accounting Policies (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Oct. 06, 2017
USD ($)
Oct. 05, 2017
USD ($)
shares
Dec. 31, 2017
USD ($)
continent
country
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2017
USD ($)
continent
country
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Number of contract drilling rigs               62    
Number of different countries having drilling contracts spanning | country     14         14    
Number of different continents having drilling contracts | continent     6         6    
Bargain purchase gain     $ 140.2         $ 140.2 $ 0.0 $ 0.0
Net foreign currency exchange gains (losses)               (5.1) (6.0) 5.4
Short-term Investments     440.0         440.0 1,442.6  
Asset Impairment Charges     $ 182.9 $ 0.0 $ 0.0 $ 0.0   $ 182.9 $ 0.0 $ 2,746.4
Gain (Loss) on Contract Termination             $ 205.0      
Antidilutive share options excluded from computation of diluted earnings per share | shares               2,000,000 500,000 800,000
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%    
Deferred Mobilization Costs [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Other assets     $ 39.9         $ 39.9 $ 43.9  
Deferred Regulatory Certification And Compliance Costs [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Other assets     15.3         15.3 14.9  
Mobilization Revenue [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Other Liabilities     35.7         35.7 62.1  
Capital Reimbursements [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Other Liabilities     $ 87.4         $ 87.4 $ 165.2  
Atwood Oceanics, Inc.                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Cash settlement for business acquisitions   $ 11.1                
Consideration transferred $ 770.7 781.8                
Cash and cash equivalents $ 445.4                  
Payments to extinguish acquiree liabilities   $ 1,300.0                
Atwood Oceanics, Inc. | Common Class A [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
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               11    
Moored Semisubmersible Rigs [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               38    
Construction in Progress [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Number of contract drilling rigs               3    
Drilling rigs and equipment [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Asset Impairment Charges                   $ 2,600.0
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               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    
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    
ENSCO DS-9 [Member] | Sales Revenue, Net [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Gain (Loss) on Contract Termination               $ 185.0    
ENSCO 8503 [Member] | Sales Revenue, Net [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Gain (Loss) on Contract Termination               20.0    
ENSCO DS-4 and ENSCO DS-9 [Member] | Sales Revenue, Net [Member]                    
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                    
Gain (Loss) on Contract Termination               $ 129.0