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

Disclosure of entity's operating segments [Abstract]

No definition available.

Name:	agi_DisclosureofentitysoperatingsegmentsAbstract
Namespace Prefix:	agi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration