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Mineral Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Disclosure of detailed information about property, plant and equipment [line items] |  |  |
Construction in progress |  | $ 98.7 | $ 203.7
Island Gold [Member] |  |  |
Disclosure of detailed information about property, plant and equipment [line items] |  |  |
Consideration paid (received) | $ 54.8 |  |

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- Definition The amount of consideration paid or received in respect of both obtaining and losing control of subsidiaries or other businesses. [Refer: Subsidiaries [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 7 -IssueDate 2020-01-01 -Paragraph 40 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2020-en-r&anchor=para_40_a&doctype=Standard -URIDate 2020-01-01 + Details
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Balance Type: | credit
Period Type: | duration

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- Definition The amount of expenditure capitalised during the construction of non-current assets that are not yet available for use. [Refer: Non-current assets] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 16 -IssueDate 2020-01-01 -Paragraph 37 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=16&code=ifrs-tx-2020-en-r&anchor=para_37&doctype=Standard -URIDate 2020-01-01 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
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Namespace Prefix: | ifrs-full_
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Period Type: | duration

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- Details
Name: | ifrs-full_SegmentsAxis=agi_IslandGoldMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |