Loans and Allowance for Loan Losses - Changes in allowance (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Changes in the allowance
Beginning Balance	$ 31,713	$ 33,758	$ 31,572	$ 34,246
Loan charge-offs	(674)	(322)	(719)	(1,106)
Loan recoveries	334	328	520	624
Ending Balance	31,373	33,764	31,373	33,764
Unallocated
Changes in the allowance
Beginning Balance	457	823	1,180	1,378
Provision for loan losses	595	1,646	(128)	1,091
Ending Balance	1,052	2,469	1,052	2,469
Commercial
Changes in the allowance
Beginning Balance	22,551	24,934	22,084	24,968
Provision for loan losses	30	(1,793)	387	(1,260)
Loan charge-offs	(637)	(273)	(664)	(1,013)
Loan recoveries	101	276	238	449
Ending Balance	22,045	23,144	22,045	23,144
Commercial \| Commercial and industrial
Changes in the allowance
Beginning Balance	9,795	10,487	8,925	10,205
Provision for loan losses	1,085	(869)	1,856	(553)
Loan charge-offs	(637)	(273)	(664)	(477)
Loan recoveries	90	275	216	445
Ending Balance	10,333	9,620	10,333	9,620
Commercial \| Real estate construction
Changes in the allowance
Beginning Balance	810	598	783	658
Provision for loan losses	68	(11)	95	(71)
Ending Balance	878	587	878	587
Commercial \| Commercial real estate
Changes in the allowance
Beginning Balance	11,946	13,849	12,376	14,105
Provision for loan losses	(1,123)	(913)	(1,564)	(636)
Loan charge-offs				(536)
Loan recoveries	11	1	22	4
Ending Balance	10,834	12,937	10,834	12,937
Consumer
Changes in the allowance
Beginning Balance	8,705	8,001	8,308	7,900
Provision for loan losses	(625)	147	(259)	169
Loan charge-offs	(37)	(49)	(55)	(93)
Loan recoveries	233	52	282	175
Ending Balance	8,276	8,151	8,276	8,151
Consumer \| Residential \| Real estate first mortgage
Changes in the allowance
Beginning Balance	6,661	6,047	6,532	5,774
Provision for loan losses	(486)	129	(357)	402
Ending Balance	6,175	6,176	6,175	6,176
Consumer \| Residential \| Real estate junior lien
Changes in the allowance
Beginning Balance	1,400	1,288	1,295	1,373
Provision for loan losses	(134)	99	(42)	(69)
Loan recoveries	201	14	214	97
Ending Balance	1,467	1,401	1,467	1,401
Consumer \| Other revolving and installment
Changes in the allowance
Beginning Balance	644	666	481	753
Provision for loan losses	(5)	(81)	140	(164)
Loan charge-offs	(37)	(49)	(55)	(93)
Loan recoveries	32	38	68	78
Ending Balance	$ 634	$ 574	$ 634	$ 574

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

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

- 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 of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=122038336&loc=d3e74567-122707

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

+ Details

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

- 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_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
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=alrs_OtherRevolvingAndInstallmentPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: