REVENUE FROM CONTRACTS WITH CUSTOMERS REVENUE FROM CONTRACTS WITH CUSTOMERS - NARRATIVE (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Contract with customer, liability, current	$ 1,141	$ 1,141		$ 1,633
Contract with customer, liability, revenue recognized		707	$ 22
Revenue, remaining performance obligation (backlog)	$ 2,458	$ 2,458		$ 2,866
Percentage anticipated to be recognized in next 12 months	89.00%	89.00%		79.00%
Capitalized contract cost net, preconstruction	$ 199	$ 199		$ 296
Contract with customer, liability, cumulative catch-up adjustment to revenue, amount reversed	(979)	(733)
Contract with customer, liability, margin decrease, change in measure of progress	$ 750	$ 750
Minimum
Disaggregation of Revenue [Line Items]
Contract receivable retainage percentage	5.00%	5.00%
Maximum
Disaggregation of Revenue [Line Items]
Contract receivable retainage percentage	10.00%	10.00%

Contract Receivable Retainage Percentage

No definition available.

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- Definition

Contract With Customer, Liability, Margin Decrease, Change In Measure Of Progress

+ References

No definition available.

+ Details

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

- Definition

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

+ Details

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- Definition

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130551-203045

+ Details

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Period Type:	duration

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, 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=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130532-203044

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130556-203045

+ Details

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Period Type:	instant

- Definition

Percentage of remaining performance obligation to total remaining performance obligation not recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130556-203045

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- Details

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