Quarterly report pursuant to Section 13 or 15(d)

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

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Funds Held for Clients and Corporate Investments (Funds Held for Clients and Corporate Investments) (Details) - USD ($)
$ in Millions
Nov. 30, 2019
May 31, 2019
Schedule of Available-for-sale Securities [Line Items]    
Funds held for clients' money market securities and other cash equivalents, Amortized cost $ 756.7 $ 204.5
Funds held for clients' money market securities and other cash equivalents, Fair value 756.7 204.5
Available-for-sale securities, Amortized cost 2,953.4 3,601.1
Available-for-sale securities, Gross unrealized gains 40.6 22.4
Available-for-sale securities, Gross unrealized losses (1.9) (2.7)
Available-for-sale securities, Fair value 2,992.1 3,620.8
Other, Amortized cost 27.2 26.3
Other, Gross unrealized gains 3.4 1.7
Other, Gross unrealized losses (0.1) (0.3)
Other, Fair value 30.5 27.7
Total funds held for clients and corporate investments, Amortized cost 3,737.3 3,831.9
Total funds held for clients and corporate investments, Gross unrealized gains 44.0 24.1
Total funds held for clients and corporate investments, Gross unrealized losses (2.0) (3.0)
Total funds held for clients and corporate investments, Fair value 3,779.3 3,853.0
Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 52.1 5.3
Available-for-sale securities, Gross unrealized gains 0.2  
Available-for-sale securities, Gross unrealized losses (0.1)  
Available-for-sale securities, Fair value 52.2 5.3
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 684.2 442.1
Available-for-sale securities, Gross unrealized gains 12.0 5.3
Available-for-sale securities, Gross unrealized losses (0.3) (0.9)
Available-for-sale securities, Fair value 695.9 446.5
General Obligation Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 734.7 840.6
Available-for-sale securities, Gross unrealized gains 10.6 7.5
Available-for-sale securities, Gross unrealized losses   (0.3)
Available-for-sale securities, Fair value 745.3 847.8
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost [1] 23.2 25.6
Available-for-sale securities, Gross unrealized gains [1] 0.2 0.2
Available-for-sale securities, Fair value [1] 23.4 25.8
Revenue Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 491.3 545.4
Available-for-sale securities, Gross unrealized gains 6.3 4.7
Available-for-sale securities, Gross unrealized losses   (0.2)
Available-for-sale securities, Fair value 497.6 549.9
U.S. Government Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 960.4 612.5
Available-for-sale securities, Gross unrealized gains 11.3 4.7
Available-for-sale securities, Gross unrealized losses (1.5) (1.3)
Available-for-sale securities, Fair value 970.2 615.9
Variable Rate Demand Notes [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 7.5 1,129.6
Available-for-sale securities, Fair value $ 7.5 $ 1,129.6
[1] Pre-refunded municipal bonds are secured by an escrow fund of U.S. government obligations.