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Share Capital (Antidilutive Securities) (Details) - $ / shares shares in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Earnings per share [line items] |  |
Share price (cad per share) | $ 10.87 | $ 7.18
Stock options |  |
Earnings per share [line items] |  |
Antidilutive securities | 0 | 1,420

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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_
Data Type: | xbrli:sharesItemType
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: | num: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 domain 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: |