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Share Capital (Employee LTI plan and share purchase plan) (Details)	12 Months Ended
	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%

X

- Definition

Maximum Number of Shares Reserved for Issuance Under Long Term Incentive Plan

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No definition available.

+ Details

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Namespace Prefix:	agi_
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Balance Type:	na
Period Type:	duration

X

- Definition

Non-Diluted Basis Allocated to the Employee Share Purchase Plan

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No definition available.

+ Details

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Namespace Prefix:	agi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Share-Based Compensation Arrangement By Share-Based Payment Award, Maximum Employee Subscription Rate1

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No definition available.

+ Details

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Namespace Prefix:	agi_
Data Type:	num:percentItemType
Balance Type:	na
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- Definition

Share-Based Payment Arrangement, Employer Matching Contribution, Percent of Match

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No definition available.

+ Details

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Namespace Prefix:	agi_
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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.

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No definition available.

+ Details

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- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=agi_EmployeeSharePurchasePlanMember
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Period Type:

X

- Details

Name:	ifrs-full_ClassesOfShareCapitalAxis=agi_OrdinaryClassAMember
Namespace Prefix:
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Period Type: