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Acquisition of Atwood (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Oct. 05, 2017
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Oct. 06, 2017
May 26, 2017
Business Acquisition [Line Items]            
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Assets   $ 8.3 $ 8.3      
Business Combination, Intangible Liability, Due to Unfavorable Drilling Contracts       $ 60.0    
Atwood            
Business Acquisition [Line Items]            
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents         $ 445.4  
Repayments of Debt $ 1,300.0          
Business Combination, Acquired Receivables, Gross Contractual Amount         $ 64.7  
Finite-lived Intangible Assets Acquired       30.1    
Amortization of Intangible Assets   $ 4.3 $ 5.8      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill       8.2    
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months       5.6    
Finite-Lived Intangible Assets, Amortization Expense, Year Two       $ 2.6    
Atwood | Class A ordinary shares, U.S.            
Business Acquisition [Line Items]            
Share Price           $ 6.70
Shares Issued, Price Per Share $ 5.83