LOANS AND ALLOWANCE FOR LOAN LOSSES - Loans Modified in a Troubled Debt Restructuring (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) loan	Sep. 30, 2016 USD ($) loan	Sep. 30, 2017 USD ($) loan	Sep. 30, 2016 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	5	5	12	24
Pre- Modification of Outstanding Recorded Investment	$ 2,022	$ 1,504	$ 12,520	$ 10,692
Post- Modification of Outstanding Recorded Investment	$ 2,022	$ 1,504	$ 12,520	$ 10,692
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	4	4	7	16
Pre- Modification of Outstanding Recorded Investment	$ 1,520	$ 872	$ 3,441	$ 3,803
Post- Modification of Outstanding Recorded Investment	$ 1,520	$ 872	$ 3,441	$ 3,803
Mortgage warehouse
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	0	0	0	0
Pre- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 0	$ 0
Post- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 0	$ 0
Real estate \| Commercial real estate (including multi-family residential)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	1	1	3	7
Pre- Modification of Outstanding Recorded Investment	$ 502	$ 632	$ 8,783	$ 6,882
Post- Modification of Outstanding Recorded Investment	$ 502	$ 632	$ 8,783	$ 6,882
Real estate \| Commercial real estate construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	0	0	1	0
Pre- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 210	$ 0
Post- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 210	$ 0
Real estate \| 1-4 family residential (including home equity)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	0	0	1	0
Pre- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 86	$ 0
Post- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 86	$ 0
Real estate \| Residential construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	0	0	0	0
Pre- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 0	$ 0
Post- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 0	$ 0
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Contracts \| loan	0	0	0	1
Pre- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 0	$ 7
Post- Modification of Outstanding Recorded Investment	$ 0	$ 0	$ 0	$ 7

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=abtx_MortgageWarehousePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=abtx_RealEstatePorfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=abtx_CommercialRealEstateIncludingMultiFamilyResidentialButNotConstructionAndLandDevelopmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=abtx_CommercialRealEstateConstructionAndLandDevelopmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=abtx_OnetoFourFamilyResidentialIncludingHomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=abtx_ResidentialConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: