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Note 19 - Derivative Instruments - Schedule of Fair Value Hedging Instruments, Statements of Financial Performance and Financial Position, Location (Details) - Designated as Hedging Instrument [Member] - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
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Sep. 30, 2024 | Dec. 31, 2023
Mortgage-backed securities, Notional Amount |  | $ 200,000 |  | $ 600,000 |
Mortgage-backed securities, Carrying Amount |  | 201,113 |  | 600,339 |
Mortgage-backed securities, Fair Value |  | 1,113 |  | 339 |
Residential Mortgage-Backed Securities [Member] |  |  |  |  |
Mortgage-backed securities, Notional Amount |  | 200,000 | [1] | 200,000 | [2]
Mortgage-backed securities, Carrying Amount |  | 201,113 | [1] | 200,241 | [2]
Mortgage-backed securities, Fair Value |  | $ 1,113 | [1] | 241 | [2]
Mortgage Loan Pools [Member] |  |  |  |  |
Mortgage-backed securities, Notional Amount | [3] |  |  | 400,000 |
Mortgage-backed securities, Carrying Amount | [3] |  |  | 400,098 |
Mortgage-backed securities, Fair Value | [3] |  |  | $ 98 |
[1] | Includes amounts related to residential agency mortgage-backed securities currently designated as the hedged item in a fair value hedge using the portfolio layer method. At September 30, 2024, the amortized cost of the closed portfolios used in these hedging relationships was $313.0 million.
[2] | Includes amounts related to residential agency mortgage-backed securities currently designated as the hedged item in a fair value hedge using the portfolio layer method. At December 31, 2023, the amortized cost of the closed portfolios used in these hedging relationships was $323.4 million.
[3] | These amounts include the amortized cost basis of residential real estate loans that were used to designate hedging relationships in which the hedged item is the stated amount of assets in the closed portfolio anticipated to be outstanding for the designated hedged period. At December 31, 2023, the amortized cost basis of the residential real estate loans used in these hedging relationships was $687.5 million.

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- Definition Amount of cumulative increase (decrease) in fair value of hedged asset (liabilities) in fair value hedge, attributable to hedged risk. + References No definition available. + Details
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Namespace Prefix: | alrs_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1A -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Net fair value of all derivative instruments designated as fair value hedging instruments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 5 -SubTopic 10 -Topic 815 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480463/815-10-45-5 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |