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Employee Benefit Plans - Reconciliation of Level 3 Pension Assets (Details) - Defined Benefit Pension Plans - USD ($)
$ in Millions
12 Months Ended
May 31, 2018
May 31, 2017
Change in the fair value of plan assets:    
Fair value of plan assets at beginning of year $ 118.8 $ 111.0
Fair value of plan assets at end of year 134.5 118.8
Significant other unobservable inputs (Level 3)    
Change in the fair value of plan assets:    
Fair value of plan assets at beginning of year 14.1 14.1
Purchases 7.8  
Sales (3.0) (0.8)
Return on plan assets related to assets still held 1.0 0.8
Fair value of plan assets at end of year 19.9 14.1
Significant other unobservable inputs (Level 3) | Funds-of-funds    
Change in the fair value of plan assets:    
Fair value of plan assets at beginning of year 7.3 7.5
Sales   (0.8)
Return on plan assets related to assets still held 0.4 0.6
Fair value of plan assets at end of year 7.7 7.3
Significant other unobservable inputs (Level 3) | Hedge funds    
Change in the fair value of plan assets:    
Fair value of plan assets at beginning of year 6.8 6.6
Sales (3.0)  
Return on plan assets related to assets still held 0.6 0.2
Fair value of plan assets at end of year 4.4 $ 6.8
Significant other unobservable inputs (Level 3) | Insurance annuities    
Change in the fair value of plan assets:    
Purchases 7.8  
Fair value of plan assets at end of year $ 7.8