Quarterly report pursuant to Section 13 or 15(d)

Funds Held for Clients and Corporate Investments (Details 2)

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Funds Held for Clients and Corporate Investments (Details 2) (USD $)
 3 Months Ended 9 Months Ended
Feb. 28, 2013
Security
Feb. 29, 2012
Feb. 28, 2013
Security
Feb. 29, 2012
May 31, 2012
Security
Maturity Date:          
Due in one year or less, Amortized cost $ 281,300,000   $ 281,300,000    
Due in one year or less, Fair value 283,900,000   283,900,000    
Due after one year through three years, Amortized cost 719,400,000   719,400,000    
Due after one year through three years, Fair value 738,000,000   738,000,000    
Due after three years through five years, Amortized cost 644,400,000   644,400,000    
Due after three years through five years, Fair value 665,100,000   665,100,000    
Due after five years, Amortized cost 3,176,700,000   3,176,700,000    
Due after five years, Fair value 3,186,700,000   3,186,700,000    
Total, Amortized cost 4,821,800,000   4,821,800,000    
Total, Fair value 4,873,700,000   4,873,700,000   3,059,000,000
Funds Held for Clients and Corporate Investments (Textual)          
Separately Designated Demand Deposit Account 20,500,000   20,500,000   13,700,000
Net unrealized gain on available-for-sale securities 51,900,000   51,900,000   59,500,000
Number of available-for-sale securities in an unrealized loss position 43   43   35
Total, Gross unrealized losses 600,000   600,000   300,000
Gross realized gains 600,000 400,000 900,000 600,000  
Gross realized losses $ 0 $ 0 $ 0 $ 0  
Minimum maturity period for VRDNs primarily categorized as due after five years 20 years   20 years    
Maximum maturity period for VRDNs, primarily categorized as due after five years (in years) 30 years   30 years