Changes in accrued restructuring charges from December 31, 2014 to December 31, 2016 are summarized as follows (in millions):
 
 
Estimated employee separation charges
 
Estimated costs to exit
 
Total
Balance as of December 31, 2014
 
$
2.3

 
$
1.1

 
$
3.4

Charges
 
11.0

 
$
—

 
11.0

Usage
 
(8.0
)
 
(0.6
)
 
(8.6
)
Balance as of December 31, 2015
 
$
5.3

 
$
0.5

 
$
5.8

Charges
 
33.2

 
—

 
$
33.2

Usage
 
(30.4
)
 
(0.5
)
 
$
(30.9
)
Balance as of December 31, 2016
 
$
8.1

 
$
—

 
$
8.1