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Fair Value Measurements Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Investment securities available for sale:    
Investment securities available for sale, at fair value $ 5,396,812us-gaap_AvailableForSaleSecurities $ 5,250,585us-gaap_AvailableForSaleSecurities
Trading assets 35,163us-gaap_DerivativeAssets 43,728us-gaap_DerivativeAssets
Liabilities, Fair Value Disclosure [Abstract]    
Trading liabilities 37,329us-gaap_DerivativeLiabilities 46,470us-gaap_DerivativeLiabilities
Fair Value, Measurements, Recurring [Member]    
Investment securities available for sale:    
Trading assets 43,164us-gaap_DerivativeAssets
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Liabilities, Fair Value Disclosure [Abstract]    
Trading liabilities 45,818us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 998us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Obligations of state and political subdivisions (municipal securities) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 582,679us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Government-Sponsored Enterprise (GSE) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 3,730,789us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Private Label [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 2,294us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | GSE Commercial Mortgage-Related Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 1,073,893us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Other Debt And Other Equity Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 6,159us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 3,957us-gaap_AvailableForSaleSecurities
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Trading assets 0us-gaap_DerivativeAssets
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Liabilities, Fair Value Disclosure [Abstract]    
Trading liabilities 0us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 998us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Obligations of state and political subdivisions (municipal securities) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Government-Sponsored Enterprise (GSE) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Private Label [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | GSE Commercial Mortgage-Related Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Asset-backed Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Other Debt And Other Equity Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 2,959us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 5,392,655us-gaap_AvailableForSaleSecurities
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Liabilities, Fair Value Disclosure [Abstract]    
Trading liabilities 43,383us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Obligations of state and political subdivisions (municipal securities) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 582,679us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Government-Sponsored Enterprise (GSE) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 3,730,789us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Private Label [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 2,294us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | GSE Commercial Mortgage-Related Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 1,073,893us-gaap_AvailableForSaleSecurities
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Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other Debt And Other Equity Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 3,000us-gaap_AvailableForSaleSecurities
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Investment securities available for sale:    
Investment securities available for sale, at fair value 200us-gaap_AvailableForSaleSecurities
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Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Obligations of state and political subdivisions (municipal securities) [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Government-Sponsored Enterprise (GSE) [Member]    
Investment securities available for sale:    
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Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Private Label [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember		 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Asset-backed Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember		 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other Debt And Other Equity Securities [Member]    
Investment securities available for sale:    
Investment securities available for sale, at fair value $ 200us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= asb_OtherDebtAndOtherEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember		 $ 299us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByAssetClassAxis
= asb_OtherDebtAndOtherEquitySecuritiesMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember