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Valuation and Qualifying Accounts (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reserve For Accounts Receivable [Member]      
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]      
Beginning Balance $ 18.3 $ 23.3 $ 20.3
Valuation Allowances and Reserves, Charged to Cost and Expense 20.2 14.8 19.7
Valuation Allowances and Reserves, Charged to Other Accounts [1] 43.7 39.1 48.5
Deductions for Purposes for which Reserves were Created 61.1 58.9 65.2
Ending Balance 21.1 18.3 23.3
Reserve For Other Investments [Member]      
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]      
Beginning Balance 3.0 3.0 3.0
Valuation Allowances and Reserves, Charged to Cost and Expense 0.0 0.0 0.0
Valuation Allowances and Reserves, Charged to Other Accounts [1] 0.0 0.0 0.0
Deductions for Purposes for which Reserves were Created 0.0 0.0 0.0
Ending Balance $ 3.0 $ 3.0 $ 3.0
[1] Charged to Other Accounts reflects the deferral of bad debt expense to a regulatory asset.