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)

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.

No definition available.

Name:	belfa_AccumulatedOtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Namespace Prefix:	belfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant