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Debt Securities Held for which Other-Than-Temporary Impairment Loss has been Recognized in Current Period or Previously (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
OtherThanTemporary Impairment Non Credit Losses Recognized In Earnings [Line Items]    
Fair Value $ 66.0 $ 73.6
Amortized Cost Basis 74.2 96.8
Noncredit-related Losses Recognized in OCI (8.2) (23.2)
Tax Effect 3.1 8.6
Amount Recorded in OCI $ (5.1) $ (14.6)