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Summary of Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]  
Schedule for Company's allowance for doubtful accounts Changes in the Company's allowance for doubtful accounts for the years ended December 31 follows:
(in millions)
 
2017
 
2016
 
2015
Balance as of January 1
 
$
4.0

 
$
4.7

 
$
4.8

Provision charged to expense
 
1.3

 
0.3

 
0.1

Amounts acquired
 
2.0

 
0.4

 
1.5

Amounts written off, net of recoveries
 
(0.7
)
 
(1.4
)
 
(1.7
)
Balance as of December 31
 
$
6.6

 
$
4.0

 
$
4.7