Quarterly report pursuant to Section 13 or 15(d)

Other Comprehensive Income/(Loss) (Details)

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Other Comprehensive Income/(Loss) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Nov. 30, 2012
Nov. 30, 2011
Nov. 30, 2012
Nov. 30, 2011
May 31, 2012
Other comprehensive Income/(Loss):          
Unrealized gains/(losses) on available-for-sale securities, net of tax $ 2.5 $ (10.1) $ 2.2 $ (3.4)  
Reclassification adjustment for the net gain on sale of available-for-sale securities realized in net income, net of tax (0.1) (0.1) (0.2) (0.2)  
Total other comprehensive income/(loss), net of tax 2.4 (10.2) 2.0 (3.6)  
Other Comprehensive Income/(Loss) (Textual)          
Accumulated other comprehensive income 39.7   39.7   37.7
Accumulated other comprehensive income, tax amount $ 23.1   $ 23.1   $ 21.8