[Line Items] for Debt and Financing Obligations [Table]

No definition available.

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

- Definition

Line Of Credit Facility Unused Capacity Commitment Fee Percentage1

+ References

No definition available.

+ Details

Name:	agi_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage1
Namespace Prefix:	agi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Maximum Borrowing Capacity1

+ References

No definition available.

+ Details

Name:	agi_LineofCreditFacilityMaximumBorrowingCapacity1
Namespace Prefix:	agi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum Net Leverage Ratio

+ References

No definition available.

+ Details

Name:	agi_MaximumNetLeverageRatio
Namespace Prefix:	agi_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum Interest Coverage Ratio

+ References

No definition available.

+ Details

Name:	agi_MinimumInterestCoverageRatio
Namespace Prefix:	agi_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of outstanding funds that the entity is obligated to repay.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_55&doctype=Standard
-URIDate 2020-01-01

+ Details

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

- Definition

The adjustment to the basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_7&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_BorrowingsAdjustmentToInterestRateBasis
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_BorrowingsByNameAxis=agi_RevolvingCreditFacility1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=agi_LondonInterbankOfferedRateLIBOR1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=agi_RevolvingCreditFacilityDrawnAmountsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments and Risk Management (Revolving Credit Facility) (Details)	Sep. 21, 2017 USD ($)	Mar. 31, 2020 USD ($)
Revolving Credit Facility [Member]
Debt and Financing Obligations [Line Items]
Borrowings		$ 100,000,000.0
Minimum interest coverage ratio	3.0
Maximum net leverage ratio	3.5
Bottom of range [member] \| Revolving Credit Facility [Member]
Debt and Financing Obligations [Line Items]
Line of credit facility, maximum borrowing capacity	$ 400,000,000.0
Commitment fee	0.42%
Top of range [member] \| Revolving Credit Facility [Member]
Debt and Financing Obligations [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500,000,000.0
LIBOR [Member] \| Bottom of range [member] \| Revolving Credit Facility - Drawn Amounts [Member]
Debt and Financing Obligations [Line Items]
Borrowings, adjustment to interest rate basis	1.875%