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Income Taxes (Summary Of Deferred Tax Assets And Deferred Tax Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Deferred tax assets    
Property costs and asset retirement obligations $ 4.5 $ 3.7
Employee benefits 8.0 6.1
Operating leases liability 31.6 25.0
Other deferred tax assets 7.2 2.1
Total gross deferred tax assets 51.3 36.9
Deferred tax liabilities    
Accumulated depreciation and amortization (213.2) (191.2)
State deferred taxes (20.2) (27.9)
Operating leases right of use assets (31.0) (24.8)
Other deferred tax liabilities (5.3) (9.7)
Total gross deferred tax liabilities (269.7) (253.6)
Net deferred tax liabilities $ (218.4) $ (216.7)