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Investments (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Partnership
Investment
Agreement
Security
Dec. 31, 2013
Security
Dec. 31, 2012
Major Categories of Investment Income [Line Items]      
Investment Income $ 11,536us-gaap_InvestmentIncomeInterestAndDividend $ 11,238us-gaap_InvestmentIncomeInterestAndDividend $ 12,216us-gaap_InvestmentIncomeInterestAndDividend
Investment expenses (2,481)us-gaap_InvestmentIncomeInvestmentExpense (2,468)us-gaap_InvestmentIncomeInvestmentExpense (2,286)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income 9,055us-gaap_InvestmentIncomeNet 8,770us-gaap_InvestmentIncomeNet 9,930us-gaap_InvestmentIncomeNet
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]      
Applicable federal income taxes (27,913)us-gaap_AvailableForSaleSecuritiesIncomeTaxExpenseOnChangeInUnrealizedHoldingGainOrLoss (26,432)us-gaap_AvailableForSaleSecuritiesIncomeTaxExpenseOnChangeInUnrealizedHoldingGainOrLoss  
Net unrealized gains - net of tax 51,840us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax 49,089us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax  
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]      
Number of Securities, Total 767us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 516us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]      
Fair value, total 218,647us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 145,056us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue  
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]      
Gross unrealized loss, total (9,724)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss (4,584)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss  
Gain and Loss on Investment [Abstract]      
Limited partnerships - net gain 7,108us-gaap_IncomeLossFromEquityMethodInvestments 7,995us-gaap_IncomeLossFromEquityMethodInvestments 7,052us-gaap_IncomeLossFromEquityMethodInvestments
Total net gains 14,930us-gaap_RealizedInvestmentGainsLosses 23,515us-gaap_RealizedInvestmentGainsLosses 9,011us-gaap_RealizedInvestmentGainsLosses
Shareholders' equity, net of deferred federal income taxes and undistributed earnings from limited partnership 28,596bwinb_ShareholdersEquityNetOfDeferredFederalIncomeTaxesAndUndistributedEarningsFromLimitedPartnership    
Adjustments for other-than-temporary impairment [Roll Forward]      
Cumulative charges to income at beginning of year 6,770us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 7,773us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 8,178us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Writedowns based on objective and subjective criteria 689us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 742us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities 888us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities
Recovery of prior writedowns upon sale or disposal (291)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold (1,745)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold (1,293)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
Net pre-tax realized gain (loss) (398)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsPeriodIncreaseDecrease 1,003us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsPeriodIncreaseDecrease 405us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsPeriodIncreaseDecrease
Cumulative charges to income at end of year 7,168us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 6,770us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld 7,773us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld
Addition (reduction) to earnings per share from net after-tax OTI gain (loss) $ (0.02)bwinb_AdditionReductionToEarningsPerShareFromNetAfterTaxRealizedGainLoss $ 0.04bwinb_AdditionReductionToEarningsPerShareFromNetAfterTaxRealizedGainLoss $ 0.02bwinb_AdditionReductionToEarningsPerShareFromNetAfterTaxRealizedGainLoss
Unrealized gain on investments previously written down at end of the year - see note below 17,127us-gaap_UnrealizedGainLossOnInvestments 13,129us-gaap_UnrealizedGainLossOnInvestments 8,158us-gaap_UnrealizedGainLossOnInvestments
Committed funds for bridge loans 7,850bwinb_CommittedFundsForBridgeLoans    
Number of bridge loan agreements 2bwinb_NumberOfBridgeLoanAgreements    
Limited Partners' Capital Account [Line Items]      
Number of partnerships investment 2bwinb_NumberOfPartnershipsInvestment    
Market value of equity and fixed maturity securities portfolio managed by broker dealers 3,334bwinb_MarketValueOfEquityAndFixedMaturitySecuritiesPortfolioManagedByBrokerDealers 19,822bwinb_MarketValueOfEquityAndFixedMaturitySecuritiesPortfolioManagedByBrokerDealers  
Total commissions and net fees earned by broker dealers and affiliates 212bwinb_TotalCommissionsAndNetFeesEarnedByBrokerDealersAndAffiliates 239bwinb_TotalCommissionsAndNetFeesEarnedByBrokerDealersAndAffiliates 186bwinb_TotalCommissionsAndNetFeesEarnedByBrokerDealersAndAffiliates
Summarized financial information of significant limited partnership investments [Abstract]      
Total assets 1,144,247us-gaap_Assets 1,072,270us-gaap_Assets  
Regulatory deposits with various insurance departments in United States and Canada, fair value 76,406bwinb_RegulatoryDepositsFairValue 63,447bwinb_RegulatoryDepositsFairValue  
Time certificate of deposit included in short-term investments 2,966us-gaap_TimeDepositsAtCarryingValue    
Percentage of fixed maturities invested in investment grade fixed maturity (in hundredths) 85.00%bwinb_PercentageOfFixedMaturitiesInvestedInInvestmentGradeFixedMaturityInHundredths    
Total fixed maturity investments 15,430us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount    
Number of different investments in fixed maturity 5bwinb_NumberOfDifferentInvestments    
Number of different third party insurance companies issuing fixed maturity investments with guarantees 2bwinb_NumberOfDifferentThirdPartyInsuranceCompaniesIssuingFixedMaturityInvestmentsWithGuarantees    
Amount of largest exposure to a single investment 5,150bwinb_AmountOfExposureToSingleInvestment    
Fixed maturity investment below investment grade 66,817bwinb_FixedMaturityInvestmentBelowInvestmentGrade    
Percentage of fixed maturity investment to total invested assets (in hundredths) 8.80%bwinb_PercentageOfFixedMaturityInvestmentToTotalInvestedAssets    
Number of investments in diversified portfolio including catastrophe bonds 40bwinb_NumberOfInvestmentsInDiversifiedPortfolioIncludingCatastropheBonds    
Net unrealized loss position of diversified portfolio 3,188us-gaap_UnrealizedLossOnSecurities    
Limited Partnership [Member]      
Limited Partners' Capital Account [Line Items]      
Commitment to make additional contributions to various limited partnerships 2,868bwinb_CommitmentToMakeAdditionalContributionsToVariousLimitedPartnerships
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Total investment in three limited partnerships 23,000us-gaap_LimitedPartnersContributedCapital
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Number of partnerships investment 2bwinb_NumberOfPartnershipsInvestment
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Aggregate estimated value of limited partnership 46,987us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Number of directors that are executive officers, directors, and owners of the organization managing the limited partnership 4bwinb_NumberOfDirectorsThatAreExecutiveOfficersDirectorsAndOwnersOfOrganizationManagingLimitedPartnership
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Ownership interest in limited partnership, minimum (in hundredths) 5.00%bwinb_OwnershipInterestInLimitedPartnershipMinimum
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Ownership interest in limited partnership, maximum (in hundredths) 16.00%bwinb_OwnershipInterestInLimitedPartnershipMaximum
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Investment gains (losses), net of fees 7,088us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
1,154us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
2,485us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
Management fees 697us-gaap_ManagementFeesBaseRevenue
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
640us-gaap_ManagementFeesBaseRevenue
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
650us-gaap_ManagementFeesBaseRevenue
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
Performance-based fees 0us-gaap_PerformanceFees
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
18us-gaap_PerformanceFees
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
0us-gaap_PerformanceFees
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
Summarized financial information of significant limited partnership investments [Abstract]      
Total partners' capital 46,987us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= us-gaap_LimitedPartnerMember
   
Significant Limited Partnership [Member]      
Limited Partners' Capital Account [Line Items]      
Aggregate estimated value of limited partnership 542,700us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
444,337us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
559,745us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
Number of significant investments in public and private equity market in India 1bwinb_NumberOfSignificantInvestmentsInPublicAndPrivateEquityMarketInIndia
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
   
Significant investment in public and private equity markets in India, value 29,868us-gaap_InvestmentOwnedAtFairValue
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
22,692us-gaap_InvestmentOwnedAtFairValue
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
 
Estimated ownership interest in significant limited partnership investment (in hundredths) 5.00%us-gaap_SubsidiaryOfLimitedLiabilityCompanyOrLimitedPartnershipOwnershipInterest
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
   
Earnings from significant limited partnership investment 7,176us-gaap_NetIncomeLossAllocatedToLimitedPartners
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
(3,176)us-gaap_NetIncomeLossAllocatedToLimitedPartners
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
2,404us-gaap_NetIncomeLossAllocatedToLimitedPartners
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
Summarized financial information of significant limited partnership investments [Abstract]      
Total assets 565,500us-gaap_Assets
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
493,028us-gaap_Assets
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
641,071us-gaap_Assets
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
Total partners' capital 542,700us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
444,337us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
559,745us-gaap_LimitedPartnersCapitalAccount
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
Net increase (decrease) in partners' capital resulting from operations 125,700us-gaap_PartnersCapitalAccountPeriodIncreaseDecrease
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
(64,550)us-gaap_PartnersCapitalAccountPeriodIncreaseDecrease
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
60,734us-gaap_PartnersCapitalAccountPeriodIncreaseDecrease
/ us-gaap_LimitedPartnersCapitalAccountByClassAxis
= bwinb_SignificantLimitedPartnershipMember
U.S. Government Obligations [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 101,094us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
113,389us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Cost or amortized cost 101,058us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
113,348us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross unrealized gains 108us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
81us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross unrealized losses (72)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
(40)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Net unrealized gains (losses) 36bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
41bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
Available-for-sale Securities, Debt Securities, Total 101,094us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
113,389us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
Cost or amortized cost 101,058us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
113,348us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Residential Mortgage-Backed Securities [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 6,066us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
13,252us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Cost or amortized cost 5,830us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
12,058us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross unrealized gains 273us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
1,334us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross unrealized losses (37)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
(140)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Net unrealized gains (losses) 236bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
1,194bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
Available-for-sale Securities, Debt Securities, Total 6,066us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
13,252us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
Cost or amortized cost 5,830us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
12,058us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Commercial Mortgage-backed Securities [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 36,440us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
28,565us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Cost or amortized cost 36,210us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
28,406us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Gross unrealized gains 630us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
308us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Gross unrealized losses (400)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
(149)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Net unrealized gains (losses) 230bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
159bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
Available-for-sale Securities, Debt Securities, Total 36,440us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
28,565us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
Cost or amortized cost 36,210us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
28,406us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
State and Municipal Obligations [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 113,777us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
115,250us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Cost or amortized cost 113,133us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
115,278us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross unrealized gains 784us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
407us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross unrealized losses (140)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(435)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Net unrealized gains (losses) 644bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
(28)bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
Available-for-sale Securities, Debt Securities, Total 113,777us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
115,250us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
Cost or amortized cost 113,133us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
115,278us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Corporate Securities [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 166,966us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
137,215us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Cost or amortized cost 170,822us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
136,991us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Gross unrealized gains 2,005us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
3,207us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Gross unrealized losses (5,861)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
(2,983)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Net unrealized gains (losses) (3,856)bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
224bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
Available-for-sale Securities, Debt Securities, Total 166,966us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
137,215us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
Cost or amortized cost 170,822us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
136,991us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
 
Foreign Government Obligations [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 27,466us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
23,879us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Cost or amortized cost 28,332us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
23,689us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross unrealized gains 114us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
588us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross unrealized losses (980)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
(398)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Net unrealized gains (losses) (866)bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
190bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
Available-for-sale Securities, Debt Securities, Total 27,466us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
23,879us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
Cost or amortized cost 28,332us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
23,689us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Total Fixed Maturities [Member]      
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]      
Fair value 451,809us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
431,550us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Cost or amortized cost 455,385us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
429,770us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Gross unrealized gains 3,914us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
5,925us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Gross unrealized losses (7,490)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(4,145)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Net unrealized gains (losses) (3,576)bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
1,780bwinb_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]      
Number of Securities 12 months or less 591us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
451us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Number of Securities Greater than 12 months 140us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
53us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Number of Securities, Total 731us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
504us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]      
Fair value, 12 months or less 176,756us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
123,145us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Fair value, Greater than 12 months 27,667us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
18,249us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Fair value, total 204,423us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
141,394us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]      
Gross unrealized loss, less than 12 months (6,083)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(3,105)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Gross unrealized loss, 12 months or longer (1,407)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(1,040)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Gross unrealized loss, total (7,490)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(4,145)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Average unrealized loss for securities in loss position (in hundredths) 4.00%bwinb_AverageUnrealizedLossOfSecuritiesToOriginalOrAdjustedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]      
One year or less 130,407us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of one year to five years 189,182us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of five years to ten years 47,209us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of ten years 9,377us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Total maturities 376,175bwinb_AvailableForSaleSecuritiesDebtMaturitiesTotalFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Asset-backed securities 75,634us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Available-for-sale Securities, Debt Securities, Total 451,809us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
431,550us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Percentage of Available for sale Securities, Debt Maturities, Fair Value [Abstract]      
One year or less (in hundredths) 28.90%bwinb_AvailableForSaleSecuritiesDebtMaturitiesNextTwelveMonthsFairValuePercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of one year to five years (in hundredths) 41.90%bwinb_AvailableForSaleSecuritiesDebtMaturitiesYearTwoThroughFiveFairValuePercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of five years to ten years (in hundredths) 10.40%bwinb_AvailableForSaleSecuritiesDebtMaturitiesYearSixThroughTenFairValuePercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of ten years (in hundredths) 2.10%bwinb_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValuePercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Total maturities (in hundredths) 83.30%bwinb_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateTotalFairValuePercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Asset-backed securities (in hundredths) 16.70%bwinb_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValuePercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Available-for-sale Securities, Debt Securities, Total (in hundredths) 100.00%bwinb_AvailableForSaleSecuritiesDebtSecuritiesTotalPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]      
One year or less 130,322us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of one year to five years 192,990us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of five years to ten years 48,176us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of ten years 8,744us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Total maturities 380,232bwinb_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateTotalAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Asset-backed securities 75,153us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Cost or amortized cost 455,385us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
429,770us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Percentage of Available for sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]      
One year or less (in hundredths) 28.60%bwinb_AvailableForSaleSecuritiesDebtMaturitiesNextTwelveMonthsAmortizedCostBasisPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of one year to five years (in hundredths) 42.40%bwinb_AvailableForSaleSecuritiesDebtMaturitiesYearTwoThroughFiveAmortizedCostBasisPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of five years to ten years (in hundredths) 10.60%bwinb_AvailableForSaleSecuritiesDebtMaturitiesYearSixThroughTenAmortizedCostBasisPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Excess of ten years (in hundredths) 1.90%bwinb_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCostBasisPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Total maturities (in hundredths) 83.50%bwinb_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateTotalAmortizedCostPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Asset-backed securities (in hundredths) 16.50%bwinb_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCostBasisPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Total (in hundredths) 100.00%bwinb_AvailableForSaleSecuritiesDebtMaturitiesAmortizedCostBasisPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Gain and Loss on Investment [Abstract]      
Gross gains 6,480us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
7,235us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
3,860us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Gross losses (4,596)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(4,371)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(3,961)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Net gains (losses) 1,884us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
2,864us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(101)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Financial Institutions [Member]      
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]      
Fair value 25,343us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
18,850us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
 
Cost or amortized cost 10,100us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
7,780us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
 
Gross unrealized gains 15,303us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
11,171us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
 
Gross unrealized losses (60)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
(101)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
 
Net unrealized gains (losses) 15,243bwinb_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
11,070bwinb_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_FinancialInstitutionMember
 
Industrial & Miscellaneous [Member]      
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]      
Fair value 136,764us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
126,978us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
 
Cost or amortized cost 68,678us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
64,307us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
 
Gross unrealized gains 70,260us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
63,009us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
 
Gross unrealized losses (2,174)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
(338)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
 
Net unrealized gains (losses) 68,086bwinb_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
62,671bwinb_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= bwinb_IndustrialMiscellaneousMember
 
Total Equity Securities [Member]      
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]      
Fair value 162,107us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
145,828us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Cost or amortized cost 78,778us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
72,087us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross unrealized gains 85,563us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
74,180us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross unrealized losses (2,234)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(439)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Net unrealized gains (losses) 83,329bwinb_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
73,741bwinb_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]      
Number of Securities 12 months or less 33us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
10us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Number of Securities Greater than 12 months 3us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Number of Securities, Total 36us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
12us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]      
Fair value, 12 months or less 13,538us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,682us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Fair value, Greater than 12 months 686us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
1,980us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Fair value, total 14,224us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,662us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]      
Gross unrealized loss, less than 12 months (2,170)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(204)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross unrealized loss, 12 months or longer (64)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(235)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross unrealized loss, total (2,234)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(439)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gain and Loss on Investment [Abstract]      
Gross gains 7,467us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
15,374us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,191us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Gross losses (1,529)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(2,718)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
(1,131)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Net gains (losses) 5,938us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
12,656us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
2,060us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Total Available-for-Sale Securities [Member]      
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]      
Fair value 613,916us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
577,378us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
 
Cost or amortized cost 534,163us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
501,857us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
 
Gross unrealized gains 89,477us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
80,105us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
 
Gross unrealized losses (9,724)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
(4,584)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
 
Net unrealized gains (losses) 79,753us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
75,521us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SecuritiesAssetsMember
 
Interest on fixed maturities [Member]      
Major Categories of Investment Income [Line Items]      
Investment Income 8,806us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
9,023us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
10,052us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Dividends on equity securities [Member]      
Major Categories of Investment Income [Line Items]      
Investment Income 2,693us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
2,166us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
2,121us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Money Market Funds, Short term and Other [Member]      
Major Categories of Investment Income [Line Items]      
Investment Income $ 37us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= bwinb_MoneyMarketFundsShortTermAndOtherMember
$ 49us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= bwinb_MoneyMarketFundsShortTermAndOtherMember
$ 43us-gaap_InvestmentIncomeInterestAndDividend
/ us-gaap_InvestmentTypeAxis
= bwinb_MoneyMarketFundsShortTermAndOtherMember