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Share-Based Compensation - Stock Options (Details)
$ in Millions
3 Months Ended 9 Months Ended
Feb. 29, 2016
USD ($)
shares
Feb. 28, 2015
USD ($)
Feb. 29, 2016
USD ($)
plan
shares
Feb. 28, 2015
USD ($)
shares
Share-based compensation        
Number of employee stock ownership plans (plan) | plan     2  
Stock Options        
Share-based compensation        
Recognized share-based compensation expense (less than) | $ $ 0.1 $ 0.1 $ 0.1 $ 0.1
Unrecognized compensation costs, net of estimated forfeitures (less than) | $ $ 0.1   $ 0.1  
Weighted-average period over which unrecognized compensation cost is expected to be recognized (years)     1 month  
Stock options granted (in shares) | shares     0 0
2007 Plan        
Share-based compensation        
Number of awards that may be granted (in shares) | shares 0   0