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Description Of The Business And Summary Of Significant Accounting Policies (Narrative) (Details)
shares in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2015
USD ($)
Sep. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Sep. 30, 2014
USD ($)
Jun. 30, 2014
USD ($)
Mar. 31, 2014
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
segments
continent
country
service
shares
Dec. 31, 2014
USD ($)
shares
Dec. 31, 2013
USD ($)
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Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Number of contract drilling rigs                   64    
Number of rigs under construction                   4    
Number of different countries having drilling contracts spanning | country                   15    
Number of different continents having drilling contracts | continent                   6    
Net foreign currency exchange gains (losses)                   $ 5,400,000 $ (2,600,000) $ 6,400,000
Short-term Investments $ 1,180,000,000       $ 757,300,000         1,180,000,000 757,300,000  
Cost of Services, Depreciation 149,700,000 $ 145,200,000 $ 140,500,000 $ 137,100,000 139,400,000 $ 135,200,000 $ 132,200,000 $ 131,100,000   $ 572,500,000 $ 537,900,000 $ 496,200,000
Number of operating segments | segments                   3    
Number of reportable segments | segments                   2    
Number of services | service                   1    
Antidilutive share options excluded from computation of diluted earnings per share | shares                   0.8 0.4 0.3
Impairment of Long Lived Assets                   $ 2,618,900,000 $ 2,463,100,000  
Deferred tax assets, noncurrent 94,800,000       63,100,000         94,800,000 63,100,000  
Deferred tax liabilities, noncurrent 4,400,000       162,900,000         4,400,000 162,900,000  
Impairment of Long-Lived Assets Held-for-use 2,460,300,000       517,300,000         2,470,300,000 1,220,800,000  
New Accounting Pronouncement, Early Adoption, Effect [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Deferred tax assets, current         (43,800,000)           (43,800,000)  
Deferred tax assets, noncurrent         43,800,000           43,800,000  
Deferred tax liabilities, current         2,500,000           2,500,000  
Deferred tax liabilities, noncurrent         2,500,000           2,500,000  
Deferred Mobilization Costs [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Other assets 77,000,000       95,700,000         77,000,000 95,700,000  
Deferred Regulatory Certification And Compliance Costs [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Other assets $ 21,200,000       $ 20,000,000         $ 21,200,000 $ 20,000,000  
Ultra Deepwater Drillships [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Number of contract drilling rigs                   10    
Semisubmersible Rigs [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Number of contract drilling rigs                   13    
Moored Semisubmersible Rigs [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Number of contract drilling rigs                   3    
Jackup Rigs [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Number of contract drilling rigs                   42    
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                   $ 98,300,000    
Other Nonrecurring Income                   18,400,000    
Ensco DS-4 [Member] | Sales Revenue, Net [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Gain (Loss) on Contract Termination                   $ 110,600,000.0    
Minimum [Member] | Drilling rigs and equipment [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Property, Plant and Equipment, Useful Life                   4 years    
Minimum [Member] | Building and Building Improvements [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Property, Plant and Equipment, Useful Life                   2 years    
Minimum [Member] | Other Equipment And Computer Hardware Software [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Property, Plant and Equipment, Useful Life                   3 years    
Maximum [Member] | Drilling rigs and equipment [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Property, Plant and Equipment, Useful Life                   35 years    
Maximum [Member] | Building and Building Improvements [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Property, Plant and Equipment, Useful Life                   30 years    
Maximum [Member] | Other Equipment And Computer Hardware Software [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Property, Plant and Equipment, Useful Life                   6 years    
Change in useful lives [Member] | Scenario, Forecast [Member] | Change in depreciation expense [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Cost of Services, Depreciation                 $ 20,000,000      
Impairment impact [Member] | Scenario, Forecast [Member] | Change in depreciation expense [Member]                        
Description Of The Business And Summary Of Significant Accounting Policies [Line Items]                        
Cost of Services, Depreciation                 $ 170,000,000