Quarterly report pursuant to Section 13 or 15(d)

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

v3.19.3.a.u2
Funds Held for Clients and Corporate Investments (Narrative) (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Nov. 30, 2019
USD ($)
security
Nov. 30, 2018
USD ($)
Nov. 30, 2019
USD ($)
security
Nov. 30, 2018
USD ($)
May 31, 2019
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]          
Net unrealized gains on available-for-sale securities $ 38.7   $ 38.7   $ 19.7
Number of available-for-sale securities in an unrealized loss position | security 113   113   269
Gross unrealized losses on available-for-sale securities $ 1.9   $ 1.9   $ 2.7
Realized gains on available-for-sale securities 0.9 $ 0.1 1.8 $ 0.2  
Realized losses on available-for-sale securities 0.0 $ 0.4 0.0 $ 0.4  
U.S. Government Agency Securities [Member]          
Schedule of Available-for-sale Securities [Line Items]          
Gross unrealized losses on available-for-sale securities $ 1.5   $ 1.5   $ 1.3
Available-for-sale securities contractual maturities     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