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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock-Based Compensation Expense by Type of Award
Total stock-based compensation expense by type of award was as follows:
 
Three months ended June 30,
 
Six months ended June 30,
 
2018
 
2017
 
2018
 
2017
 
(In millions)
Stock appreciation rights
$
1.4

 
$
—

 
$
0.5

 
$
2.4

Stock options
—

 
0.3

 
0.1

 
0.7

Restricted shares, service based
1.2

 
1.2

 
2.2

 
2.4

Restricted shares, performance based
2.5

 
1.8

 
3.6

 
4.4

Employee stock purchase plan
0.1

 
0.1

 
0.3

 
0.3

Total stock-based compensation expense
$
5.2

 
$
3.4

 
$
6.7

 
$
10.2