Quarterly report pursuant to Section 13 or 15(d)

Comprehensive Income (Details)

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Comprehensive Income (Details) (USD $)
In Millions
3 Months Ended
Aug. 31, 2011
Aug. 31, 2010
May 31, 2011
Comprehensive Income [Abstract]      
Net income $ 148.9 $ 131.9  
Other comprehensive income:      
Unrealized gains on available-for-sale securities, net of tax 6.7 11.9  
Reclassification adjustment for the net gain on sale of available-for-sale securities realized in net income, net of tax 0 (0.1)  
Total other comprehensive income 6.7 11.8  
Total comprehensive income 155.6 143.7  
Comprehensive Income (Textuals) [Abstract]      
Accumulated other comprehensive income 44.2   37.5
Accumulated other comprehensive income, tax amount $ 25.5   $ 21.7