Loans and Allowance for Credit Losses - Changes in allowance (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Changes in the allowance
Beginning balance	$ 35,696	$ 31,373	$ 31,146	$ 31,572
Provision for credit losses			460
Loan charge-offs	(151)	(747)	(516)	(1,466)
Loan recoveries	745	342	1,343	862
Ending balance	36,290	30,968	36,290	30,968
Provision for off-balance sheet credit loss			44
Debt securities held-to-maturity credit loss			46
Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			3,857
Unallocated
Changes in the allowance
Beginning balance	1,727	1,052	268	1,180
Provision for credit losses	(774)	(100)	(31)	(228)
Ending balance	953	952	953	952
Unallocated \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			716
Commercial
Changes in the allowance
Beginning balance	24,424	22,045	23,292	22,084
Provision for credit losses	1,235	868	878	1,255
Loan charge-offs	(134)	(672)	(394)	(1,336)
Loan recoveries	467	282	984	520
Ending balance	25,992	22,523	25,992	22,523
Commercial \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			1,232
Commercial \| Commercial and industrial
Changes in the allowance
Beginning balance	7,813	10,333	9,158	8,925
Provision for credit losses	442	(845)	(275)	1,011
Loan charge-offs	(134)	(672)	(394)	(1,336)
Loan recoveries	456	105	950	321
Ending balance	8,577	8,921	8,577	8,921
Commercial \| Commercial and industrial \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			(862)
Commercial \| Real estate construction
Changes in the allowance
Beginning balance	3,646	878	1,446	783
Provision for credit losses	1,063	378	745	473
Loan recoveries		76		76
Ending balance	4,709	1,332	4,709	1,332
Commercial \| Real estate construction \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			2,518
Commercial \| Commercial real estate
Changes in the allowance
Beginning balance	12,965	10,834	12,688	12,376
Provision for credit losses	(270)	1,335	408	(229)
Loan recoveries	11	101	34	123
Ending balance	12,706	12,270	12,706	12,270
Commercial \| Commercial real estate \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			(424)
Consumer
Changes in the allowance
Beginning balance	9,545	8,276	7,586	8,308
Provision for credit losses	(461)	(768)	(387)	(1,027)
Loan charge-offs	(17)	(75)	(122)	(130)
Loan recoveries	278	60	359	342
Ending balance	9,345	7,493	9,345	7,493
Consumer \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			1,909
Consumer \| Real estate first mortgage
Changes in the allowance
Loan charge-offs			(9)
Consumer \| Real estate junior lien
Changes in the allowance
Loan charge-offs			(77)
Consumer \| Residential \| Real estate first mortgage
Changes in the allowance
Beginning balance	7,901	6,175	5,769	6,532
Provision for credit losses	(389)	(584)	(339)	(941)
Loan charge-offs	(9)		(9)
Loan recoveries	254		256
Ending balance	7,757	5,591	7,757	5,591
Consumer \| Residential \| Real estate first mortgage \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			2,080
Consumer \| Residential \| Real estate junior lien
Changes in the allowance
Beginning balance	1,351	1,467	1,289	1,295
Provision for credit losses	(14)	(109)	140	(151)
Loan charge-offs			(77)
Loan recoveries		7	52	221
Ending balance	1,337	1,365	1,337	1,365
Consumer \| Residential \| Real estate junior lien \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			(67)
Consumer \| Other revolving and installment
Changes in the allowance
Beginning balance	293	634	528	481
Provision for credit losses	(58)	(75)	(188)	65
Loan charge-offs	(8)	(75)	(36)	(130)
Loan recoveries	24	53	51	121
Ending balance	$ 251	$ 537	251	$ 537
Consumer \| Other revolving and installment \| Adoption of CECL \| Cumulative Effect, Period of Adoption, Adjustment
Changes in the allowance
Beginning balance			$ (104)

Amount of increase in allowance for credit loss on accounts receivable, from recovery.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_AllowanceForDoubtfulAccountsReceivableRecoveries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of expense (reversal of expense) for expected credit loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

+ References

+ Details

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

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_CommercialBorrowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=us-gaap_ConsumerBorrowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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