Note 3 - Revenue - Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Contract assets - current (unbilled receivable)	$ 14,135	$ 16,318
Contract liabilities - current (deferred revenue)	2,077	$ 653
Balance, January 1	653
New advance payments received	7,798
Recognized as revenue during period	(6,380)
Currency translation	6
Balance, December 31	$ 2,077

The amount of advance payments received from customers for goods or services to be provided at a later date.

No definition available.

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Namespace Prefix:	belfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of change in contract with customer liability due to currency translation adjustment.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized during period from contract with customer.

+ References

No definition available.

+ Details

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Namespace Prefix:	belfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130531-203044

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=121556615&loc=SL49130531-203044

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- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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+ Details

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