Leases (Right of Use) (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Disclosure of leases [Abstract]
Opening balance - December 31, 2018	$ 7.9
ROU asset additions	2.4
At December 31, 2019	10.3
Opening balance - December 31, 2018	3.6
Amortization	1.1
At December 31, 2019	4.7
Net carrying value	$ 5.6

Accumulated Amortization, Right of Use Assets

No definition available.

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Namespace Prefix:	agi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Disclosure of leases [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Right of Use Assets, Net

+ References

No definition available.

+ Details

Name:	agi_RightofUseAssetsNet
Namespace Prefix:	agi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of depreciation of right-of-use assets. [Refer: Depreciation and amortisation expense; Right-of-use assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 53
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_53_a&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_DepreciationRightofuseAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of assets that represent a lessee's right to use an underlying asset for the lease term. Underlying asset is an asset that is the subject of a lease, for which the right to use that asset has been provided by a lessor to a lessee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 53
-Subparagraph j
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_53_j&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The increase (decrease) in the revaluation surplus that relates to right-of-use assets. [Refer: Revaluation surplus; Right-of-use assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2019-01-01
-Paragraph 57
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2019-en-r&anchor=para_57&doctype=Standard
-URIDate 2019-03-27

+ Details

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