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
Aug. 31, 2018
May 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Funds held for clients' money market securities and other cash equivalents, Amortized cost $ 1,179.8 $ 1,942.3
Funds held for clients' money market securities and other cash equivalents, Gross unrealized gains  
Funds held for clients' money market securities and other cash equivalents, Gross unrealized losses  
Funds held for clients' money market securities and other cash equivalents, Fair value 1,179.8 1,942.3
Available-for-sale securities, Amortized cost 2,947.5 3,143.1
Available-for-sale securities, Gross unrealized gains 2.9 2.4
Available-for-sale securities, Gross unrealized losses (38.8) (40.7)
Available-for-sale securities, Fair value 2,911.6 3,104.8
Other, Amortized cost 17.4 16.1
Other, Gross unrealized gains 2.5 2.1
Other, Gross unrealized losses  
Other, Fair value 19.9 18.2
Total funds held for clients and corporate investments, Amortized cost 4,144.7 5,101.5
Total funds held for clients and corporate investments, Gross unrealized gains 5.4 4.5
Total funds held for clients and corporate investments, Gross unrealized losses (38.8) (40.7)
Total funds held for clients and corporate investments, Fair value 4,111.3 5,065.3
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 367.7 328.6
Available-for-sale securities, Gross unrealized gains 0.3 0.1
Available-for-sale securities, Gross unrealized losses (7.3) (7.3)
Available-for-sale securities, Fair value 360.7 321.4
General Obligation Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,149.8 1,231.6
Available-for-sale securities, Gross unrealized gains 1.3 1.3
Available-for-sale securities, Gross unrealized losses (9.7) (11.2)
Available-for-sale securities, Fair value 1,141.4 1,221.7
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 45.7 50.9 [1]
Available-for-sale securities, Gross unrealized gains 0.2 0.1 [1]
Available-for-sale securities, Gross unrealized losses (0.1) (0.1) [1]
Available-for-sale securities, Fair value 45.8 50.9 [1]
Revenue Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 767.5 813.5
Available-for-sale securities, Gross unrealized gains 1.0 0.9
Available-for-sale securities, Gross unrealized losses (7.1) (8.0)
Available-for-sale securities, Fair value 761.4 806.4
U.S. Government Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 454.5 410.2
Available-for-sale securities, Gross unrealized gains 0.1
Available-for-sale securities, Gross unrealized losses (14.6) (14.1)
Available-for-sale securities, Fair value 440.0 396.1
Variable Rate Demand Notes [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 162.3 308.3
Available-for-sale securities, Gross unrealized gains  
Available-for-sale securities, Gross unrealized losses  
Available-for-sale securities, Fair value $ 162.3 $ 308.3
[1] Pre-refunded municipal bonds are secured by an escrow fund of U.S. government obligations.