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Shareholders' Equity Shareholders' Equity (Share Issuance Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Oct. 06, 2017
Oct. 05, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares Issuance [Line Items]                
Stock Issued During Period, Shares, New Issues     65,550,000   1,822,432   1,822,432  
Shares Issued, Price Per Share     $ 9.25          
Proceeds from (Repurchase of) Equity     $ 585.5          
Gain (Loss) on Extinguishment of Debt       $ 6.2   $ (2.6) $ 287.8 $ (33.5)
Atwood Oceanics, Inc.                
Shares Issuance [Line Items]                
Business Combination, Consideration Transferred $ 770.7 $ 781.8            
Class A Ordinary Shares, U.S. [Member] | Atwood Oceanics, Inc.                
Shares Issuance [Line Items]                
Number of ordinary shares   132,200,000            
Shares Issued, Price Per Share   $ 5.83            
Senior Notes [Member]                
Shares Issuance [Line Items]                
Debt Instrument, Repurchased Face Amount         $ 1,130.6 194.1 1,130.6  
Gain (Loss) on Extinguishment of Debt           $ 18.2    
Senior Notes [Member] | 5.75% Senior notes due 2044                
Shares Issuance [Line Items]                
Debt Instrument, Repurchased Face Amount         $ 24.5   24.5  
Gain (Loss) on Extinguishment of Debt             $ 8.8