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Financial Instruments and Risk Management (Unrealized Gains (Losses) on Foreign Currency Contracts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about hedged items [line items]
Unrealized gains	$ (0.7)	$ 0.6

X

- Definition

Unrealized Gain (Loss) on Derivatives1

+ References

No definition available.

+ Details

Name:	agi_UnrealizedGainLossonDerivatives1
Namespace Prefix:	agi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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

Name:	ifrs-full_DisclosureOfDetailedInformationAboutHedgedItemsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration