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Allowance for Doubtful Accounts (Tables)
 12 Months Ended
Dec. 31, 2018
Text Block [Abstract]  
Summary of Allowance for Doubtful Accounts

The allowance for doubtful accounts consisted of the following (in millions):

 

 

 

December 31,

 

 

 

2018

 

 

2017

 

 

2016

 

Balance at beginning of year

 

$

0.9

 

 

$

0.7

 

 

$

0.8

 

Provision charged to expense

 

 

1.2

 

 

 

0.2

 

 

 

—

 

Write-offs, less recoveries

 

 

—

 

 

 

—

 

 

 

(0.1

)

Balance at end of year

 

$

2.1

 

 

$

0.9

 

 

$

0.7