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Pension and other post-retirement benefits - Narrative (Details) - CAD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of benefit plans [line items]    
Description of Retirement benefit plan Under the supplemental executive pension plan (SEPP), Cameco provides a lump sum benefit equal to the present value of a lifetime pension benefit based on the executive’s length of service and final average earnings.  
Description of Retirement benefits promised to participants The plan provides for unreduced benefits to be paid at the normal retirement age of 65, however unreduced benefits could be paid if the executive was at least 60 years of age and had 20 years of service at retirement. This program provides for a benefit determined by a formula based on earnings and service, reduced by the benefits payable under the registered base plan.  
2018 [Member]    
Disclosure of benefit plans [line items]    
Contributions and fees payable $ 1,713,000  
Pension plans [member]    
Disclosure of benefit plans [line items]    
Weighted Average Length Of Time Of Defined Benefit Obligation 20 years 3 months 18 days 19 years 7 months 6 days
Other Benefit Plans [Member]    
Disclosure of benefit plans [line items]    
Weighted Average Length Of Time Of Defined Benefit Obligation 15 years 8 months 12 days 15 years 2 months 12 days