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Financial Instruments and Risk Management (Unrealized Gains (Losses) on Foreign Currency Contracts) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Disclosure of detailed information about hedged items [line items] |  |
Unrealized gains | $ 0.6 | $ 0.4

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- Definition Unrealized Gain (Loss) on Derivatives1 + References No definition available. + Details
Name: | agi_UnrealizedGainLossonDerivatives1
Namespace Prefix: | agi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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_DisclosureOfDetailedInformationAboutHedgedItemsLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration