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Financial Instruments and Risk Management (Available for Sale Financial Assets) (Details) $ in Millions | Dec. 31, 2018 USD ($)
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Monarques Gold Corp [Member] |
Investment [Line Items] |
Financial assets available-for-sale | $ 7.3
Other Investments [Member] |
Investment [Line Items] |
Financial assets available-for-sale | $ 0.5

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- Definition The amount of non-derivative financial assets that are designated as available for sale or are not classified as (a) loans and receivables; (b) held-to-maturity investments; or (c) financial assets at fair value through profit or loss. [Refer: Derivative financial assets; Financial assets at fair value through profit or loss; Held-to-maturity investments] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Expiry date 2021-01-01 -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 8 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_8_d&doctype=Standard&book=b -URIDate 2018-03-16 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_CounterpartiesAxis=agi_MonarquesGoldCorpMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |