FAIR VALUE OF FINANCIAL INSTRUMENTS - Schedule of Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Financial Assets:
Available for sale securities, at fair value	$ 2,240,834	$ 1,783,551
Fair Value, Recurring
Financial Assets:
Other assets - Cash flow hedges	1,068	1,754
Total assets carried at fair value	2,241,902	1,785,305
Financial liabilities:
Other liabilities - Cash flow hedges	749
Total liabilities carried at fair value	749
Fair Value, Recurring \| Level 1
Financial Assets:
Other assets - Cash flow hedges	0	0
Total assets carried at fair value	10,234	8,048
Financial liabilities:
Other liabilities - Cash flow hedges	0
Total liabilities carried at fair value	0
Fair Value, Recurring \| Level 2
Financial Assets:
Other assets - Cash flow hedges	1,068	1,754
Total assets carried at fair value	2,025,366	1,573,755
Financial liabilities:
Other liabilities - Cash flow hedges	749
Total liabilities carried at fair value	749
Fair Value, Recurring \| Level 3
Financial Assets:
Other assets - Cash flow hedges	0	0
Total assets carried at fair value	206,302	203,502
Financial liabilities:
Other liabilities - Cash flow hedges	0
Total liabilities carried at fair value	0
GSE certificates & CMOs
Financial Assets:
Available for sale securities, at fair value	706,003	567,070
GSE certificates & CMOs \| Fair Value, Recurring
Financial Assets:
Available for sale securities, at fair value	706,003	567,070
GSE certificates & CMOs \| Fair Value, Recurring \| Level 1
Financial Assets:
Available for sale securities, at fair value	0	0
GSE certificates & CMOs \| Fair Value, Recurring \| Level 2
Financial Assets:
Available for sale securities, at fair value	706,003	567,070
GSE certificates & CMOs \| Fair Value, Recurring \| Level 3
Financial Assets:
Available for sale securities, at fair value	0	0
Non-GSE certificates & CMOs
Financial Assets:
Available for sale securities, at fair value	448,157	273,232
Non-GSE certificates & CMOs \| Fair Value, Recurring
Financial Assets:
Available for sale securities, at fair value	448,157	273,232
Non-GSE certificates & CMOs \| Fair Value, Recurring \| Level 1
Financial Assets:
Available for sale securities, at fair value	0	0
Non-GSE certificates & CMOs \| Fair Value, Recurring \| Level 2
Financial Assets:
Available for sale securities, at fair value	448,157	273,232
Non-GSE certificates & CMOs \| Fair Value, Recurring \| Level 3
Financial Assets:
Available for sale securities, at fair value	0	0
Asset-Backed Securities ("ABS")
Financial Assets:
Available for sale securities, at fair value	784,193	629,168
Asset-Backed Securities ("ABS") \| Fair Value, Recurring
Financial Assets:
Available for sale securities, at fair value	784,193	629,168
Asset-Backed Securities ("ABS") \| Fair Value, Recurring \| Level 1
Financial Assets:
Available for sale securities, at fair value	0	0
Asset-Backed Securities ("ABS") \| Fair Value, Recurring \| Level 2
Financial Assets:
Available for sale securities, at fair value	784,193	629,168
Asset-Backed Securities ("ABS") \| Fair Value, Recurring \| Level 3
Financial Assets:
Available for sale securities, at fair value	0	0
Corporate
Financial Assets:
Available for sale securities, at fair value	85,945	95,504
Corporate \| Fair Value, Recurring
Financial Assets:
Available for sale securities, at fair value	85,945	95,504
Corporate \| Fair Value, Recurring \| Level 1
Financial Assets:
Available for sale securities, at fair value	0	0
Corporate \| Fair Value, Recurring \| Level 2
Financial Assets:
Available for sale securities, at fair value	85,945	95,504
Corporate \| Fair Value, Recurring \| Level 3
Financial Assets:
Available for sale securities, at fair value	0	0
Other
Financial Assets:
Available for sale securities, at fair value	10,234	15,075
Other \| Fair Value, Recurring
Financial Assets:
Available for sale securities, at fair value	10,234	15,075
Other \| Fair Value, Recurring \| Level 1
Financial Assets:
Available for sale securities, at fair value	10,234	8,048
Other \| Fair Value, Recurring \| Level 2
Financial Assets:
Available for sale securities, at fair value	0	7,027
Other \| Fair Value, Recurring \| Level 3
Financial Assets:
Available for sale securities, at fair value	0	0
Residential PACE assessments
Financial Assets:
Available for sale securities, at fair value	206,302	203,502
Residential PACE assessments \| Fair Value, Recurring
Financial Assets:
Available for sale securities, at fair value	206,302	203,502
Residential PACE assessments \| Fair Value, Recurring \| Level 1
Financial Assets:
Available for sale securities, at fair value	0	0
Residential PACE assessments \| Fair Value, Recurring \| Level 2
Financial Assets:
Available for sale securities, at fair value	0	0
Residential PACE assessments \| Fair Value, Recurring \| Level 3
Financial Assets:
Available for sale securities, at fair value	$ 206,302	$ 203,502

Fair value portion of asset recognized for present right to economic benefit.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of all asset derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

+ Details

Name:	us-gaap_CashFlowHedgeDerivativeInstrumentAssetsAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of all liability derivatives designated as cash flow hedging instruments.

+ References

+ Details

Name:	us-gaap_CashFlowHedgeDerivativeInstrumentLiabilitiesAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=amal_TraditionalSecuritiesIssuedByUSGovernmentSponsoredEntitiesAndCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=amal_TraditionalSecuritiesAndCollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=amal_PropertyAssessedCleanEnergyAssessmentSecuritiesResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: