Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

No definition available.

+ Details

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

- Definition

The fair value of financial assets. [Refer: At fair value [member]; Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 25
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_25&doctype=Standard
-URIDate 2024-03-27

+ Details

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

- Definition

The fair value of financial liabilities. [Refer: At fair value [member]; Financial liabilities]

+ References

+ Details

Name:	ifrs-full_FinancialLiabilitiesAtFairValue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	ifrs-full_HedgingInstrumentsAxis=agi_CollarsandForwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialAssetsAxis=ifrs-full_DerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=agi_CurrencyHedgingDerivativeInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=agi_FuelOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=agi_GoldOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments and Risk Management (Fair Value And Presentation in the Balance Sheet Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Collars and Forwards
Disclosure of detailed information about financial instruments [line items]
Financial liabilities, at fair value		$ 6.6
Derivatives \| Currency Hedging Derivative Instruments
Disclosure of detailed information about financial instruments [line items]
Financial liabilities, at fair value	$ (9.0)
Derivatives \| Fuel Options
Disclosure of detailed information about financial instruments [line items]
Financial assets, at fair value	0.1	$ 0.2
Derivatives \| Gold Options
Disclosure of detailed information about financial instruments [line items]
Financial assets, at fair value	$ (0.8)