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Stock-Based Compensation (Tables)
3 Months Ended
Aug. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan
The following table summarizes stock-based compensation expense included in Selling, general and administrative expenses for the periods indicated: 
 
Three months ended
 
August 31,
 
August 31,
 
2016
 
2015
Stock option expense
$
0.8

 
$
0.7

Restricted stock unit expense
0.7

 
0.6

Management stock purchase plan
0.0

 
0.0

Employee stock purchase plan
0.1

 
0.1

Total stock-based compensation expense
$
1.6

 
$
1.4