The amount of cash and cash equivalents deposited in financial institutions.

No definition available.

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Namespace Prefix:	bgm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of cash and cash equivalents deposited in financial institution subject to annual limit.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

The amount of cash and cash equivalents as of the balance sheet date that is not insured.

+ References

No definition available.

+ Details

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

- Definition

Maximum amount of cash, cash equivalents and restricted cash deposited at financial institution in PRC that were protected under deposit protection scheme per depositor.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the number of customer.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of vendor.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of percentage of accounts payable outstanding from transaction with supplier.

+ References

No definition available.

+ Details

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Namespace Prefix:	bgm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of cash deposited in financial institutions as of the balance sheet date that is insured by the Federal Deposit Insurance Corporation.

+ References

No definition available.

+ Details

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

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478785/954-310-50-2

+ Details

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Data Type:	xbrli:stringItemType
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Period Type:	duration

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-21

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-20

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-20

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Period Type:	duration

- Details

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- Details

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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_CostOfGoodsTotalMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_AccountsReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=bgm_CustomerOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=bgm_CustomerTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_MajorCustomersAxis=bgm_CustomerFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
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- Details

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentrations and Credit Risk (Details)	6 Months Ended
	Mar. 31, 2025 USD ($) customer item	Mar. 31, 2024 item customer	Mar. 31, 2023 item customer	Mar. 31, 2025 CNY (¥) customer item	Mar. 31, 2025 HKD ($) customer item	Sep. 30, 2024 USD ($)	Sep. 30, 2024 CNY (¥)	Sep. 30, 2024 HKD ($)
Concentration Risk
Cash and cash equivalents, certificates of deposit and restricted cash were on deposit at financial institutions not subject to insured	$ 8,981,076					$ 6,902,275
Cash and cash equivalents deposited in financial institutions	733,672					1,058,635
Cash and cash equivalents deposited in financial institutions annual limit	65,000				$ 500,000	65,000		$ 500,000
Cash deposited in FI insured by FDIC	$ 981,801					$ 1,856,344
Cash equivalents and restricted cash protection \| ¥				¥ 500,000			¥ 500,000
Customer \| Revenue
Concentration Risk
Number of customers \| customer	2	2	2	2	2
Customer \| Purchases
Concentration Risk
Number of vendors \| item	2	2	0	2	2
Customer \| Accounts Receivable
Concentration Risk
Number of customers \| customer	2	2		2	2
Number of vendors \| item	1	0		1	1
Customer \| Customer One \| Revenue
Concentration Risk
Concentration risk (in percent)	16.00%	16.00%	16.00%
Customer \| Customer One \| Accounts Receivable
Concentration Risk
Concentration risk (in percent)	35.00%	58.00%
Customer \| Customer Two \| Revenue
Concentration Risk
Concentration risk (in percent)	11.00%	14.00%	15.00%
Customer \| Customer Four \| Accounts Receivable
Concentration Risk
Concentration risk (in percent)	32.00%	15.00%
Supplier \| Supplier One \| Purchases
Concentration Risk
Concentration risk (in percent)	16.00%	13.00%	10.00%
Supplier \| Supplier One \| Accounts Payable
Concentration Risk
Percentage of accounts payable outstanding from transaction with supplier	47.00%	10.00%
Supplier \| Supplier Two \| Purchases
Concentration Risk
Concentration risk (in percent)	10.00%	13.00%