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

No definition available.

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- Definition

Line Of Credit Facility Unused Capacity Commitment Fee Percentage1

+ References

No definition available.

+ Details

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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.

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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.

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Data Type:	xbrli:pureItemType
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- Definition

Minimum Interest Coverage Ratio

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No definition available.

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Name:	agi_MinimumInterestCoverageRatio
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Data Type:	xbrli:pureItemType
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- Definition

The amount of interest and other costs that an entity incurs in connection with the borrowing of funds.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_BorrowingCostsIncurred
Namespace Prefix:	ifrs-full_
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Balance Type:	na
Period Type:	duration

- 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 2018-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_7&doctype=Standard
-URIDate 2018-03-16

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_BorrowingsByNameAxis=agi_RevolvingCreditFacility1Member
Namespace Prefix:
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Balance Type:
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- Details

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- Details

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- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=agi_LondonInterbankOfferedRateLIBOR1Member
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- Details

Name:	ifrs-full_BorrowingsByNameAxis=agi_RevolvingCreditFacilityDrawnAmountsMember
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Financial Instruments and Risk Management (Revolving Credit Facility) (Details)		12 Months Ended
	Sep. 21, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Revolving Credit Facility [Member]
Debt and Financing Obligations [Line Items]
Line of credit facility, maximum borrowing capacity	$ 400,000,000.0
Minimum interest coverage ratio	3.0
Maximum net leverage ratio	3.5
Borrowing costs incurred		$ 800,000	$ 2,100,000
Bottom of range [member] \| Revolving Credit Facility [Member]
Debt and Financing Obligations [Line Items]
Commitment fee	0.45%
Top of range [member] \| Revolving Credit Facility [Member]
Debt and Financing Obligations [Line Items]
Commitment fee	0.70%
LIBOR [Member] \| Bottom of range [member] \| Revolving Credit Facility - Drawn Amounts [Member]
Debt and Financing Obligations [Line Items]
Borrowings, adjustment to interest rate basis	2.00%
LIBOR [Member] \| Top of range [member] \| Revolving Credit Facility - Drawn Amounts [Member]
Debt and Financing Obligations [Line Items]
Borrowings, adjustment to interest rate basis	3.125%