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 6 Months Ended
Nov. 30, 2018
USD ($)
security
Nov. 30, 2017
USD ($)
Nov. 30, 2018
USD ($)
security
Nov. 30, 2017
USD ($)
May 31, 2018
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]          
Net unrealized losses on available-for-sale securities $ 45.3   $ 45.3   $ 38.3
Number of available-for-sale securities in an unrealized loss position | security 1,090   1,090   970
Gross unrealized losses on available-for-sale securities $ 46.2   $ 46.2   $ 40.7
Realized losses on available-for-sale securities $ 0.4 $ 0.0 $ 0.4 $ 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