Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Funds Held for Clients and Corporate Investments (Narrative) (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Nov. 30, 2017
USD ($)
security
Nov. 30, 2017
USD ($)
security
May 31, 2017
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]      
Net unrealized loss on available-for-sale securities $ 14.2 $ 14.2  
Net unrealized gain on available-for-sale securities     $ 32.0
Number of available-for-sale securities in an unrealized loss position | security 689 689 216
Gross unrealized losses on available-for-sale securities $ 22.7 $ 22.7 $ 5.8
Realized losses on available-for-sale securities $ 0.0 $ 0.0  
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