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Share-Based Compensation - Performance Restricted Stock Units - Narrative (Details) - Performance Restricted Stock Units
shares in Thousands, $ in Millions
3 Months Ended 6 Months Ended
Nov. 30, 2016
USD ($)
shares
Aug. 31, 2016
shares
Nov. 30, 2015
USD ($)
Nov. 30, 2016
USD ($)
performance_metric
shares
Nov. 30, 2015
USD ($)
May 31, 2016
shares
Share-based compensation            
Nonvested shares outstanding (in shares) 337     337   328
Fiscal 2016 Grants            
Share-based compensation            
Nonvested shares outstanding additional shares authorized in period (in shares)   19        
Nonvested shares outstanding reduction in period (in shares) 26          
Nonvested number of remaining shares authorized (in shares) 21     21    
Recognized share-based compensation expense (benefit) | $ $ 0.3   $ 0.2 $ 0.9 $ 0.8  
Unrecognized compensation cost | $ $ 4.7     $ 4.7    
Nonvested shares outstanding (in shares) 337     337    
Weighted-average period over which unrecognized compensation cost is expected to be recognized (years)       3 years 10 months 24 days    
Fiscal 2016 Grants | Accrued Expenses and Other Liabilities            
Share-based compensation            
Aggregate liability related to outstanding discretionary PRSUs, less than | $ $ 0.1     $ 0.1    
Executive and Senior Officers            
Share-based compensation            
Performance period (years)       1 year    
Number of performance award metrics | performance_metric       3    
Requisite service period (years)       5 years