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Investment Securities - Proceeds from the sale of held-to-maturity investment securities (Details) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 USD ($) item | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) item | Sep. 30, 2020 USD ($)
Investment Securities |  |  |  |
Number of held to maturity securities sold | item | 1 |  | 1 |
Held-to-maturity security amortized cost | $ 330 |  | $ 330 |
Proceeds | 348 | $ 0 | 1,763 | $ 0
Realized gains | $ 11 |  | $ 11 |

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- Definition Amount of investment in debt security measured at amortized costsold during the period. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of gain on sale of investment in debt security measured at amortized cost (held-to-maturity), including other-than-temporary impairment (OTTI). + References No definition available. + Details
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Balance Type: | credit
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- Definition Number of held to maturity securities sold during the period. + References No definition available. + Details
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Namespace Prefix: | alrs_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The cash inflow associated with the sale of securities that had been designated as held-to-maturity. Excludes proceeds from maturities, prepayments and calls by the issuer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26853-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585 + Details
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