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Benefit Plans (Summary Of Non-Vested Share Award Related Compensation Expense Recognized) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Tax benefit $ (4.8) $ (5.9) $ (4.8)
Total non-vested share award related compensation expense included in net income 28.0 30.6 32.5
Contract Drilling [Member]      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Non-vested share award related compensation expense 18.3 19.9 19.5
General And Administrative [Member]      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Non-vested share award related compensation expense 14.5 16.6 17.8
Operating Expenses [Member]      
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]      
Non-vested share award related compensation expense $ 32.8 $ 36.5 $ 37.3