Quarterly report pursuant to Section 13 or 15(d)

Funds Held for Clients and Corporate Investments (Narrative) (Details)

v3.10.0.1
Funds Held for Clients and Corporate Investments (Narrative) (Details)
$ in Millions
 3 Months Ended
Aug. 31, 2018
USD ($)
security
Aug. 31, 2018
USD ($)
security
May 31, 2018
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]      
Net unrealized losses on available-for-sale securities $ 35.9 $ 35.9 $ 38.3
Number of available-for-sale securities in an unrealized loss position | security 916 916 970
Gross unrealized losses on available-for-sale securities $ 38.8 $ 38.8 $ 40.7
Maximum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Short-term investments maturity at acquisition 90 days    
Variable Rate Demand Notes [Member] | Minimum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities contractual maturities   20 years  
Variable Rate Demand Notes [Member] | Maximum [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale securities contractual maturities   30 years