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Note 18 - Accumulated Other Comprehensive Loss - Accumulated Other Comprehensive Loss (Details) (Parentheticals) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Unrealized loss on marketable securities arising during the period, tax | $ 0 | $ 0
Change in unfunded SERP liability, tax | $ (1,377) | $ (639)

X
- Definition Represents the tax expense (benefit) of arising from the accumulated appreciation or loss in value of total available-for-sale securities at the end of an accounting period. + References No definition available. + Details
Name: | belfa_AccumulatedOtherComprehensiveIncomeLossAvailableforsaleSecuritiesAdjustmentTax
Namespace Prefix: | belfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Represents the tax portion in relation to net gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other post-retirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. + References No definition available. + Details
Name: | belfa_AccumulatedOtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Namespace Prefix: | belfa_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant