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Investment Securities (Details 7) (Debt securities issued by corporations, USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Investment securities    
Fair value investments $ 2,332.4 $ 2,135.1
AA
   
Investment securities    
Fair value investments 210.6 206.8
A
   
Investment securities    
Fair value investments 842.4 802.8
BBB
   
Investment securities    
Fair value investments 1,262.5 1,110.8
BB
   
Investment securities    
Fair value investments 7.4 6.2
Other rating
   
Investment securities    
Fair value investments $ 9.5 $ 8.5