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Financial instruments - Narrative (Details) - CAD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Nov. 16, 2020 | Dec. 31, 2018
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Gain (loss) | $ 36,577,000 | $ 32,269,000 |  |
Cash and cash equivalents | 918,382,000 | 1,062,431,000 |  | $ 711,528,000
Short-term investments | 24,985,000 | $ 0 |  |
Pledged cash as security | $ 190,140,000 |  |  |
Bottom of range [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Quoted market yields | 0.30% | 1.70% |  |
Top of range [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Quoted market yields | 1.10% | 1.80% |  |
Interest rate swap contracts [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Gain (loss) | $ 5,977,000 | $ 2,068,000 |  |
Credit risk [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Cash and cash equivalents | 918,382,000 | 1,062,431,000 |  |
Short-term investments | $ 24,985,000 | $ 0 |  |
Interest rate risk [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Proportion of outstanding debt that carries fixed rate | 100.00% | 85.00% |  |
Interest rate risk [member] | Interest rate swap contracts [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Fair value of Cameco's interest rate swap assets | $ 0 | $ 2,313,000 | $ 7,330,000 |
Interest rate risk [member] | Interest rate swap contracts [member] | Series E senior unsecured debenture [Member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Notional amount |  | $ 150,000,000 |  |
Interest rate risk [member] | Interest rate swap contracts [member] | Series E senior unsecured debenture [Member] | Floating interest rate [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Interest rate basis |  | the three-month Canada Dealer Offered Rate |  |
Margin rate |  | 1.20% |  |
Interest rate risk [member] | Interest rate swap contracts [member] | Series E senior unsecured debenture [Member] | Fixed interest rate [member] |  |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |  |
Interest rate |  | 3.75% |  |

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