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, 2016
May 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Funds held for clients money market securities and other cash equivalents, Amortized cost $ 245.2 $ 502.4
Funds held for clients money market securities and other cash equivalents, Fair value 245.2 502.4
Available-for-sale securities, Amortized cost 3,932.5 4,094.3
Available-for-sale securities, Gross unrealized gains 63.5 48.3
Available-for-sale securities, Gross unrealized losses (0.5) (0.7)
Available-for-sale securities, Fair value 3,995.5 4,141.9
Other, Amortized cost 15.6 14.2
Other, Gross unrealized gains 1.0 0.8
Other, Gross unrealized losses   (0.1)
Other, Fair value 16.6 14.9
Total funds held for clients and corporate investments, Amortized cost 4,193.3 4,610.9
Total funds held for clients and corporate investments, Gross unrealized gains 64.5 49.1
Total funds held for clients and corporate investments, Gross unrealized losses (0.5) (0.8)
Total funds held for clients and corporate investments, Fair value 4,257.3 4,659.2
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 142.9 128.4
Available-for-sale securities, Gross unrealized gains 4.7 2.9
Available-for-sale securities, Gross unrealized losses   (0.1)
Available-for-sale securities, Fair value 147.6 131.2
General Obligation Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 1,559.0 1,594.1
Available-for-sale securities, Gross unrealized gains 35.6 27.6
Available-for-sale securities, Gross unrealized losses (0.1) (0.1)
Available-for-sale securities, Fair value 1,594.5 1,621.6
Pre-Refunded Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost [1] 58.1 60.2
Available-for-sale securities, Gross unrealized gains [1] 1.5 1.4
Available-for-sale securities, Fair value [1] 59.6 61.6
Revenue Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 981.1 916.2
Available-for-sale securities, Gross unrealized gains 21.1 15.8
Available-for-sale securities, Gross unrealized losses (0.1) (0.2)
Available-for-sale securities, Fair value 1,002.1 931.8
U.S. Government Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 219.1 160.8
Available-for-sale securities, Gross unrealized gains 0.6 0.6
Available-for-sale securities, Gross unrealized losses (0.3) (0.3)
Available-for-sale securities, Fair value 219.4 161.1
Variable Rate Demand Notes [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized cost 972.3 1,234.6
Available-for-sale securities, Fair value $ 972.3 $ 1,234.6
[1] Pre-refunded municipal bonds are secured by an escrow fund of U.S. government obligations.