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v3.19.1

Share Capital (Employee LTI plan and share purchase plan) (Details) | 12 Months Ended
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Dec. 31, 2018
Employee Share Purchase Plan [Member] |
Disclosure of terms and conditions of share-based payment arrangement [line items] |
Employee contribution rate | 10.00%
Employer match | 75.00%
Ordinary Class A [Member] |
Disclosure of terms and conditions of share-based payment arrangement [line items] |
Maximum number of shares that may be reserved and set aside for issuance under the long-term incentive plan | 6.50%
Issued and outstanding shares (on a non-diluted basis) specifically allocated to the employee share purchase plan | 0.20%

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- Definition Maximum Number of Shares Reserved for Issuance Under Long Term Incentive Plan + References No definition available. + Details
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- Definition Non-Diluted Basis Allocated to the Employee Share Purchase Plan + References No definition available. + Details
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Namespace Prefix: | agi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Share-Based Compensation Arrangement By Share-Based Payment Award, Maximum Employee Subscription Rate1 + References No definition available. + Details
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Namespace Prefix: | agi_
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Balance Type: | na
Period Type: | instant

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- Definition Share-Based Payment Arrangement, Employer Matching Contribution, Percent of Match + References No definition available. + Details
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Namespace Prefix: | agi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_ClassesOfShareCapitalAxis=agi_OrdinaryClassAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |