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Securities Narrative (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Security
Dec. 31, 2013
Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]      
Proceeds from sales of securities $ 58,900,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds $ 24,200,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds $ 41,000,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Gross gains realized on sale of securities 770,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 732,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 743,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross loss realized on sale of securities 584,000us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 314,000us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 298,000us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Net impairment losses 0us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments
Tax expense related to realized gains 63,000us-gaap_OtherTaxExpenseBenefit 142,000us-gaap_OtherTaxExpenseBenefit 151,000us-gaap_OtherTaxExpenseBenefit
Carrying value of securities pledged for fiduciary powers $ 125,700,000us-gaap_AvailableForSaleSecuritiesRestricted $ 102,300,000us-gaap_AvailableForSaleSecuritiesRestricted  
Number of securities temporarily impaired 118mbrg_NumberOfSecuritiesTemporarilyImpaired    
Number of securities temporarily impaired in investment grade 116mbrg_NumberOfSecuritiesTemporarilyImpairedInInvestmentGrade    
Number of securities temporarily impaired in speculative grade 2mbrg_NumberOfSecuritiesTemporarilyImpairedInSpeculativeGrade