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Summary of Significant Accounting Policies (Stock-Based Compensation Plans) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award      
Total stock compensation expense $ 1.4us-gaap_AllocatedShareBasedCompensationExpense $ 1.4us-gaap_AllocatedShareBasedCompensationExpense $ 1.3us-gaap_AllocatedShareBasedCompensationExpense
Omnibus Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award      
Shares reserved for future issuance 700,000us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= bmi_OmnibusIncentivePlanMember