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Investments - Narrative (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Sep. 30, 2014
Investments [Abstract]      
Aggregate carrying values of investment securities pledged as collateral $ 5.7ben_AggregateCarryingValuesOfInvestmentSecuritiesPledgedAsCollateralForOutstandingFederalHomeLoanBankBorrowing   $ 6.1ben_AggregateCarryingValuesOfInvestmentSecuritiesPledgedAsCollateralForOutstandingFederalHomeLoanBankBorrowing
Other-than-temporary impairment of investments 1.0us-gaap_ImpairmentOfInvestments 0.6us-gaap_ImpairmentOfInvestments  
Other-than-temporary impairment of other investments   $ 0.2us-gaap_CostmethodInvestmentsOtherThanTemporaryImpairment