XML 21 R85.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Financial Instruments and Risk Management (Fuel Option Contracts) (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Jan. 07, 2019 | Dec. 31, 2019
Disclosure of detailed information about hedged items [line items] |  |
Gains (losses) on change in fair value of derivatives | $ 3.9 |
Fuel Options [Member] | Commodity price risk [member] |  |
Disclosure of detailed information about hedged items [line items] |  |
Gains (losses) on change in fair value of derivatives |  | $ (0.5)

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

X
- Definition The gains (losses) resulting from change in the fair value of derivatives recognised in profit or loss. [Refer: Derivatives [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 85 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_85&doctype=Standard -URIDate 2019-03-27 + Details
Name: | ifrs-full_GainsLossesOnChangeInFairValueOfDerivatives
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | ifrs-full_HedgingInstrumentsAxis=agi_FuelOptionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ifrs-full_TypesOfRisksAxis=ifrs-full_CommodityPriceRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |