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Mineral Property, Plant and Equipment (Narrative) (Details) $ in Millions |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Apr. 03, 2024 USD ($) shares | Apr. 03, 2024 USD ($) $ / shares shares | May 23, 2023 USD ($) shares | Sep. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2024 USD ($) $ / shares | Dec. 31, 2023 USD ($) | Dec. 31, 2023 USD ($) $ / shares
Disclosure of detailed information about property, plant and equipment [line items] |  |  |  |  |  |  |  |
Reversal of impairment expense |  |  |  | $ 57.1 |  |  |  |
Share price (cad per share) | $ / shares |  |  |  |  |  | $ 26.52 |  | $ 17.74
Depreciation, right-of-use assets |  |  |  |  | $ 6.2 |  |  |
Interest capitalized to mineral, property, plant and equipment |  |  |  |  | $ 7.7 |  | $ 0.0 |
Capitalisation rate of borrowing costs eligible for capitalisation |  |  |  |  | 7.03% |  |  |
Construction in progress |  |  |  |  | $ 417.9 | $ 417.9 | 299.0 | $ 299.0
Phase 3+ Expansion Project |  |  |  |  |  |  |  |
Disclosure of detailed information about property, plant and equipment [line items] |  |  |  |  |  |  |  |
Interest capitalized to mineral, property, plant and equipment |  |  |  |  | $ 11.3 |  | $ 0.0 |
Capitalisation rate of borrowing costs eligible for capitalisation |  |  |  |  | 7.04% |  |  |
Right of Use Assets |  |  |  |  |  |  |  |
Disclosure of detailed information about property, plant and equipment [line items] |  |  |  |  |  |  |  |
Right-of-use assets |  |  |  |  | $ (41.5) | $ (41.5) |  |
Orford Mining Corporation |  |  |  |  |  |  |  |
Disclosure of detailed information about property, plant and equipment [line items] |  |  |  |  |  |  |  |
Number of shares iss | shares | 0.005588 |  |  |  |  |  |  |
Number of shares in entity held by entity (in shares) | shares | 61,660,902 | 61,660,902 |  |  |  |  |  |
Percentage of shares held by entity | 27.50% |  |  |  |  |  |  |
Common shares issued as part of consideration | shares | 908,689 | 908,689 |  |  |  |  |  |
Share price (cad per share) | $ / shares |  | $ 19.87 |  |  |  |  |  |
Total consideration | $ 20.7 | $ 20.7 |  |  |  |  |  |
Transaction costs | $ 1.0 |  |  |  |  |  |  |
Manitou Gold Inc. |  |  |  |  |  |  |  |
Disclosure of detailed information about property, plant and equipment [line items] |  |  |  |  |  |  |  |
Total consideration |  |  | $ 16.7 |  |  |  |  |
Transaction costs |  |  | $ 0.2 |  |  |  |  |
Number of Alamos shares for each Manitou share held | shares |  |  | 0.0035251 |  |  |  |  |
Ownership interest (in shares) | shares |  |  | 65,211,077 |  |  |  |  |
Ownership interest, common stock (percentage) |  |  | 0.19 |  |  |  |  |
Weighted average cost of capital, measurement input [member] |  |  |  |  |  |  |  |
Disclosure of detailed information about property, plant and equipment [line items] |  |  |  |  |  |  |  |
Weighted average cost of capital |  |  |  |  | 0.05 | 0.05 |  |

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