Annual report pursuant to Section 13 and 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
May 31, 2017
May 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Funds held for clients money market securities and other cash equivalents, Amortized cost $ 264.8 $ 502.4
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 264.8 502.4
Available-for-sale securities, Amortized cost 4,581.2 4,094.3
Available-for-sale securities, Gross unrealized gains 37.8 48.3
Available-for-sale securities, Gross unrealized losses (5.8) (0.7)
Available-for-sale securities, Fair value 4,613.2 4,141.9
Other, Amortized cost 14.8 14.2
Other, Gross unrealized gains 1.9 0.8
Other, Gross unrealized losses (0.1)
Other, Fair value 16.7 14.9
Total funds held for clients and corporate investments, Amortized cost 4,860.8 4,610.9
Total funds held for clients and corporate investments, Gross unrealized gains 39.7 49.1
Total funds held for clients and corporate investments, Gross unrealized losses (5.8) (0.8)
Total funds held for clients and corporate investments, Fair value 4,894.7 4,659.2
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 208.6 128.4
Available-for-sale securities, Gross unrealized gains 2.7 2.9
Available-for-sale securities, Gross unrealized losses (0.5) (0.1)
Available-for-sale securities, Fair value 210.8 131.2
General Obligation Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,422.0 1,594.1
Available-for-sale securities, Gross unrealized gains 21.2 27.6
Available-for-sale securities, Gross unrealized losses (0.9) (0.1)
Available-for-sale securities, Fair value 1,442.3 1,621.6
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost [1] 54.6 60.2
Available-for-sale securities, Gross unrealized gains [1] 0.9 1.4
Available-for-sale securities, Gross unrealized losses [1]
Available-for-sale securities, Fair value [1] 55.5 61.6
Revenue Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 929.2 916.2
Available-for-sale securities, Gross unrealized gains 12.5 15.8
Available-for-sale securities, Gross unrealized losses (0.8) (0.2)
Available-for-sale securities, Fair value 940.9 931.8
U.S. Government Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 328.9 160.8
Available-for-sale securities, Gross unrealized gains 0.5 0.6
Available-for-sale securities, Gross unrealized losses (3.6) (0.3)
Available-for-sale securities, Fair value 325.8 161.1
Variable Rate Demand Notes [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,637.9 1,234.6
Available-for-sale securities, Gross unrealized gains
Available-for-sale securities, Gross unrealized losses
Available-for-sale securities, Fair value $ 1,637.9 $ 1,234.6
[1] Pre-refunded municipal bonds are secured by an escrow fund of U.S. government obligations.