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, 2017
May 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Funds held for clients money market securities and other cash equivalents, Amortized cost $ 1,194.6 $ 264.8
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 1,194.6 264.8
Available-for-sale securities, Amortized cost 4,318.9 4,581.2
Available-for-sale securities, Gross unrealized gains 42.2 37.8
Available-for-sale securities, Gross unrealized losses (3.6) (5.8)
Available-for-sale securities, Fair value 4,357.5 4,613.2
Other, Amortized cost 15.8 14.8
Other, Gross unrealized gains 2.2 1.9
Other, Gross unrealized losses
Other, Fair value 18.0 16.7
Total funds held for clients and corporate investments, Amortized cost 5,529.3 4,860.8
Total funds held for clients and corporate investments, Gross unrealized gains 44.4 39.7
Total funds held for clients and corporate investments, Gross unrealized losses (3.6) (5.8)
Total funds held for clients and corporate investments, Fair value 5,570.1 4,894.7
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 263.9 208.6
Available-for-sale securities, Gross unrealized gains 3.6 2.7
Available-for-sale securities, Gross unrealized losses (0.3) (0.5)
Available-for-sale securities, Fair value 267.2 210.8
General Obligation Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,407.4 1,422.0
Available-for-sale securities, Gross unrealized gains 23.2 21.2
Available-for-sale securities, Gross unrealized losses (0.4) (0.9)
Available-for-sale securities, Fair value 1,430.2 1,442.3
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost [1] 60.9 54.6
Available-for-sale securities, Gross unrealized gains [1] 1.1 0.9
Available-for-sale securities, Gross unrealized losses [1]
Available-for-sale securities, Fair value [1] 62.0 55.5
Revenue Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 921.3 929.2
Available-for-sale securities, Gross unrealized gains 13.5 12.5
Available-for-sale securities, Gross unrealized losses (0.2) (0.8)
Available-for-sale securities, Fair value 934.6 940.9
U.S. Government Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 368.6 328.9
Available-for-sale securities, Gross unrealized gains 0.8 0.5
Available-for-sale securities, Gross unrealized losses (2.7) (3.6)
Available-for-sale securities, Fair value 366.7 325.8
Variable Rate Demand Notes [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,296.8 1,637.9
Available-for-sale securities, Gross unrealized gains
Available-for-sale securities, Gross unrealized losses
Available-for-sale securities, Fair value $ 1,296.8 $ 1,637.9
[1] Pre-refunded municipal bonds are secured by an escrow fund of U.S. government obligations.