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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Aggregate allowance for doubtful accounts $ 42.8 $ 45.8  
Asset retirement obligation $ 55.5 $ 53.5  
Stock Options, SARs and RSUs      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Aggregate number of shares excluded from computation of EPS 62.5 58.7 53.7
PSUs      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Aggregate number of shares excluded from computation of EPS 23.9 9.2 6.2