Quarterly report pursuant to Section 13 or 15(d)

Comprehensive Income (Details)

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Comprehensive Income (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Feb. 29, 2012
Feb. 28, 2011
Feb. 29, 2012
Feb. 28, 2011
May 31, 2011
Comprehensive Income [Abstract]          
Net income $ 135.4 $ 130.6 $ 424.7 $ 396.4  
Other comprehensive income/(loss):          
Unrealized gains/(losses) on available-for-sale securities, net of tax 9.4 (9.7) 6.0 (12.8)  
Reclassification adjustment for the net gain on sale of available-for-sale securities realized in net income, net of tax (0.2) (0.1) (0.4) (0.3)  
Total other comprehensive income/(loss) 9.2 (9.8) 5.6 (13.1)  
Total comprehensive income 144.6 120.8 430.3 383.3  
Comprehensive Income (Textual) [Abstract]          
Accumulated other comprehensive income 43.1   43.1   37.5
Accumulated other comprehensive income, tax amount $ 24.9   $ 24.9   $ 21.7