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Inventory (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Disclosure of inventories [Abstract] |  |
Mining and processing costs, inventories | $ 360.4 | $ 319.9
Amortization costs, Inventories |  | 152.7
Indirect Production Costs |  | 5.4
Reclassified Amortization Costs |  | $ 1.1

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- Definition Amortization Costs, Inventories + References No definition available. + Details
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- Definition Disclosure of inventories [Abstract] + References No definition available. + Details
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- Definition Indirect Production Costs + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Mining and Processing Costs, Inventories + References No definition available. + Details
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Namespace Prefix: | agi_
Data Type: | xbrli:monetaryItemType
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- Definition Reclassified Amortization Costs + References No definition available. + Details
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Namespace Prefix: | agi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration