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v3.25.1

Share Capital (Antidilutive Securities) (Details) - $ / shares shares in Thousands | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023
Earnings per share [line items] |  |
Share price (cad per share) | $ 26.52 | $ 17.74
Employee Stock Option1 |  |
Earnings per share [line items] |  |
Antidilutive securities | 5 |

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- Definition Antidilutive Securities Excluded From Computation Of Earnings Per Share, Amount1 + References No definition available. + Details
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Namespace Prefix: | agi_
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Balance Type: | na
Period Type: | duration

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- Definition Share Price1 + References No definition available. + Details
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Namespace Prefix: | agi_
Data Type: | dtr-types1:perShareItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table. + References No definition available. + Details
Name: | ifrs-full_EarningsPerShareLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | agi_AntidilutiveSecuritiesAxis=agi_EmployeeStockOption1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |