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Note 15 - Subsequent Event (Details Textual) - MEXICO - USD ($) $ in Millions	May 05, 2020	Apr. 29, 2020	Mar. 31, 2020
Minimum [Member]
Normal Production Rate			$ 4.0
Maximum [Member]
Normal Production Rate			$ 4.5
Subsequent Event [Member] | COVID-19 Pandemic [Member]
Facility Production, Percentage of Normal Production Rate	60.00%	20.00%

X

- Definition

The normal production rate for the facility.

+ References

No definition available.

+ Details

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

X

- Definition

The current facility production expressed as a percentage of normal production rate.

+ References

No definition available.

+ Details

Name:	belfa_FacilityProductionPercentageOfNormalProductionRate
Namespace Prefix:	belfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	srt_StatementGeographicalAxis=country_MX
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_UnusualOrInfrequentItemAxis=belfa_Covid19PandemicMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: