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Asset Retirement Obligations - Additional Information (Details) - Asset retirement obligation - CAD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of other provisions [line items]      
Estimated cash flows, undiscounted amount $ 721,000 $ 714,800  
Estimated inflation rate 1.50% 1.40%  
Estimated risk free rate 1.20% 1.80%  
Other provisions $ 760,383 $ 667,974 $ 646,898