Annual report pursuant to Section 13 and 15(d)

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

v3.7.0.1
Funds Held for Clients and Corporate Investments (Narrative) (Details)
$ in Millions
 12 Months Ended
May 31, 2017
USD ($)
security
May 31, 2016
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]    
Net unrealized gain on available-for-sale securities $ 32.0 $ 47.6
Number of available-for-sale securities in an unrealized loss position | security 216 63
Gross unrealized losses on available-for-sale securities $ 5.8 $ 0.7
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