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Acquisition of Richmond Mines Ltd. (Narrative) (Details) - Richmont Mines Ltd [Member] $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Nov. 23, 2017 USD ($) shares | Dec. 31, 2017 USD ($) | Dec. 31, 2017 USD ($)
Disclosure of detailed information about business combination [line items] |  |  |
Conversion of Richmont units to Alamos units (note 5) | shares | 88,398,804 |  |
Number of shares issued, business combination, value | $ 615.3 |  |
Transaction costs | 3.8 |  |
Share based compensation, acquisitions | $ 12.1 |  |
Revenue of acquiree since acquisition date |  | $ 14.9 |
Profit of acquiree since acquisition date |  | $ 1.5 |
Revenue of combined entity as if combination occurred at beginning of period |  |  | $ 125.3
Profit (loss) of combined entity as if combination occurred at beginning of period |  |  | $ 12.0
Ordinary shares [member] |  |  |
Disclosure of detailed information about business combination [line items] |  |  |
Conversion ratio of shares | 1.385 |  |
Other Loss [Member] |  |  |
Disclosure of detailed information about business combination [line items] |  |  |
Share based compensation, acquisitions | $ 1.3 |  |

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