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VALUATION AND QUALIFYING ACCOUNTS - Activity (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts [Member]      
Movement in Valuation Allowances and Reserves [Roll Forward]      
Beginning Balance $ 17.7 $ 16.5 $ 14.3
Provision for Doubtful Accounts 25.1 19.5 14.1
Write-Offs (21.9) (18.3) (11.9)
Ending Balance 20.9 17.7 16.5
Estimated Revenue Adjustments [Member]      
Movement in Valuation Allowances and Reserves [Roll Forward]      
Beginning Balance 4.1 4.0 3.1
Provision for Estimated Revenue Adjustments 14.4 7.9 6.1
Write-Offs (12.3) (7.8) (5.2)
Ending Balance $ 6.2 $ 4.1 $ 4.0