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Accounting Changes - Narrative (Details) - CAD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2020
Jun. 30, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of changes in accounting estimates [line items]                
Assets       $ 9,747   $ 9,747 $ 9,508  
Depreciation and amortization           $ 654 590 $ 574
Impairment loss $ 70              
Coal generation | Maximum                
Disclosure of changes in accounting estimates [line items]                
Useful life measured as period of time, property, plant and equipment           29 years    
Coal generation | Minimum                
Disclosure of changes in accounting estimates [line items]                
Useful life measured as period of time, property, plant and equipment           1 year    
Wind and Solar                
Disclosure of changes in accounting estimates [line items]                
Useful life measured as period of time, property, plant and equipment         30 years      
Wind and Solar | Maximum                
Disclosure of changes in accounting estimates [line items]                
Useful life measured as period of time, property, plant and equipment   15 years            
Wind and Solar | Minimum                
Disclosure of changes in accounting estimates [line items]                
Useful life measured as period of time, property, plant and equipment   10 years            
Change in Useful Lives Estimate | Sheerness                
Disclosure of changes in accounting estimates [line items]                
Depreciation and amortization             (8)  
Change in Useful Lives Estimate | Coal generation                
Disclosure of changes in accounting estimates [line items]                
Depreciation and amortization             16  
Change in Useful Lives Estimate | Wind and Solar                
Disclosure of changes in accounting estimates [line items]                
Depreciation and amortization             11  
Decommissioning and restoration | Centralia Coal Mine                
Disclosure of changes in accounting estimates [line items]                
Provision for decommissioning, restoration and rehabilitation costs             141  
Impairment loss             $ 141  
Alberta Mine | Mining assets | Gross carrying amount                
Disclosure of changes in accounting estimates [line items]                
Assets       373   $ 373    
Alberta Mine | Change in Useful Lives Estimate                
Disclosure of changes in accounting estimates [line items]                
Depreciation and amortization               $ 38
Alberta Mine | Change in Useful Lives Estimate | Mining assets                
Disclosure of changes in accounting estimates [line items]                
Depreciation and amortization       15        
Alberta Mine | Decommissioning and restoration                
Disclosure of changes in accounting estimates [line items]                
Provision for decommissioning, restoration and rehabilitation costs 75              
Sundance Unit 2                
Disclosure of changes in accounting estimates [line items]                
Impairment loss     $ 38          
Sundance Unit 2 | Change in Useful Lives Estimate                
Disclosure of changes in accounting estimates [line items]                
Impairment loss     $ 38          
Sundance Unit 3 | Change in Useful Lives Estimate                
Disclosure of changes in accounting estimates [line items]                
Impairment loss $ 70              
Sarnia | Decommissioning and restoration                
Disclosure of changes in accounting estimates [line items]                
Provision for decommissioning, restoration and rehabilitation costs       $ 15   $ 15