Annual report pursuant to Section 13 and 15(d)

Employee Benefit Plans (Narrative) (Details)

Employee Benefit Plans (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
May 31, 2017
May 31, 2016
May 31, 2015
Deferred Compensation Plans [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Percentage of annual base salary and bonus that eligible employees may defer 50.00%    
Percentage of Board cash compensation that eligible outside directors may defer 100.00%    
Amounts accrued under deferred compensation plans $ 16.7 $ 14.9  
401 (K) Plan [Member]      
Defined Contribution Plan Disclosure [Line Items]      
Maximum percentage of salary deferral 50.00%    
Years of service to be eligible for matching contribution 1 year    
Minimum number of hours worked for eligibility 41 days 16 hours    
Percentage of employer matching contributions of eligible employee pay 50.00%    
Percentage of employee pay eligible for employer matching contributions 8.00%    
Company contributions to the plan $ 22.4 $ 21.4 $ 19.6
Percentage participant directed 100.00%