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Investments - Reconciliation of Other than Temporary Impairment on Fixed Maturity (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Investments, Debt and Equity Securities [Abstract]      
Beginning balance $ 2.7 $ 0.7 $ 0
Increases attributable to credit losses on securities:      
Other-than-temporary impairment was previously recognized 0 0.1 0
Other-than-temporary impairment was not previously recognized 0 1.9 0.7
Ending balance $ 2.7 $ 2.7 $ 0.7