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Segmented Information (Narrative) (Details) | Dec. 31, 2019 geographical_area
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Disclosure of entity's operating segments [Abstract] |
Number of geographical areas | 2

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- Definition Disclosure of entity's operating segments [Abstract] + References No definition available. + Details
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Namespace Prefix: | agi_
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Balance Type: | na
Period Type: | duration

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- Definition Number of Geographical Areas + References No definition available. + Details
Name: | agi_NumberofGeographicalAreas
Namespace Prefix: | agi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant