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Share-based compensation plans - Narrative (Details)
 12 Months Ended
Dec. 31, 2017
CAD ($)
shares
Number_employees
Dec. 31, 2016
CAD ($)
shares
Number_employees
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Stock options held 5,809,077 5,929,550
Options exercisable - Weighted average exercisable price | $ $ 24.95 $ 25.46
Stock Option Plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting period The options carry vesting periods of one to three years, and expire eight years from the date granted  
Authorized shares 43,017,198  
Outstanding shares 27,870,289  
Performance Share Unit Plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting period three-year vesting period  
Stock options held 1,070,997 892,895
Restricted Share Unit Plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting period The RSUs carry vesting periods of one to three years  
Stock options held 463,151 557,957
Employee Share Ownership Plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting period Shares purchased with Company contributions and with dividends paid on such shares become unrestricted 12 months from the date on which such shares were purchased  
Employers contribution per employee Under the plan, employees have the opportunity to participate in the program to a maximum of 6% of eligible earnings each year with Cameco matching the first 3% of employee-paid shares by 50%. Cameco contributes $1,000 of shares annually to each employee that is enrolled in the plan  
Number of participants | Number_employees 2,979 3,356
Total number of shares purchased 370,381 404,550
Total amount of employer's contribution | $ $ 4,473,000 $ 5,204,000
Deferred Share Unit Plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting period Each DSU fully vests upon award  
Stock options held 452,981 514,352
Description of method of settlement for share-based payment arrangement 60% of each director’s annual retainer is paid in DSUs. In addition, on an annual basis, directors can elect to receive 25%, 50%, 75% or 100% of the remaining 40% of their annual retainer and any additional fees in the form of DSUs  
Phantom Stock Option Plan [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Vesting period vest over three years and expire eight years from the date granted  
Stock options held 391,714 347,858
Options exercisable - Weighted average exercisable price | $ $ 22.13 $ 24.13
Phantom Stock Option Plan [member] | Bottom of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Exercise prices range | $ 14.7 16.38
Phantom Stock Option Plan [member] | Top of range [member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Exercise prices range | $ 39.53 39.53
Cash Settled Plan [Member]    
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Liabilities from share-based payment transactions | $ $ 5,771,000 $ 7,558,000