The amount of consideration paid or received in respect of both obtaining and losing control of subsidiaries or other businesses. [Refer: Subsidiaries [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2020-01-01
-Paragraph 40
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2020-en-r&anchor=para_40_a&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_ConsiderationPaidReceived
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of expenditure capitalised during the construction of non-current assets that are not yet available for use. [Refer: Non-current assets]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 16
-IssueDate 2020-01-01
-Paragraph 37
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=16&code=ifrs-tx-2020-en-r&anchor=para_37&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_ConstructionInProgress
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

- Details

Name:	ifrs-full_SegmentsAxis=agi_IslandGoldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Mineral Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about property, plant and equipment [line items]
Construction in progress		$ 98.7	$ 203.7
Island Gold [Member]
Disclosure of detailed information about property, plant and equipment [line items]
Consideration paid (received)	$ 54.8