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Segmented Information (Narrative) (Details)	Dec. 31, 2017 geographical_area
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

Name:	agi_DisclosureofentitysoperatingsegmentsAbstract
Namespace Prefix:	agi_
Data Type:	xbrli:stringItemType
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