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Accounting Changes - Narrative (Details) - CAD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 01, 2019
Jun. 30, 2017
Sep. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2019
Disclosure of changes in accounting estimates [line items]              
Impact of change in accounting policy (Note 3)         $ 3 $ (13)  
Right-of-use assets       $ 146 0    
Property, plant and equipment       6,207 6,164 6,578  
Finance lease liabilities $ 32            
Depreciation and amortization       590 574 635  
Contract liabilities (Note 5)       14 80    
Deferred income tax liabilities (Note 11)       472 501    
Non-controlling interests (Note 12)       1,101 1,137    
Deficit       (1,455) (1,496)    
Lease liabilities 52            
Non-cancellable operating leases 83     77 80    
Revisions in discount rates       16 (37)    
IFRS 16              
Disclosure of changes in accounting estimates [line items]              
Right-of-use assets 85     146 52    
Property, plant and equipment         38    
Assets under finance leases (29)            
Coal generation              
Disclosure of changes in accounting estimates [line items]              
Property, plant and equipment       2,120 2,172 2,457  
Deficit              
Disclosure of changes in accounting estimates [line items]              
Impact of change in accounting policy (Note 3)         3 (14)  
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       222     $ 141
Impairment loss       141      
Alberta Mine | Change in Useful Lives Estimate              
Disclosure of changes in accounting estimates [line items]              
Depreciation and amortization         38 58  
Sundance Unit 1              
Disclosure of changes in accounting estimates [line items]              
Impairment loss   $ 20          
Sundance Unit 1 | Change in Useful Lives Estimate              
Disclosure of changes in accounting estimates [line items]              
Depreciation and amortization           26  
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]              
Depreciation and amortization           4  
IFRS 15              
Disclosure of changes in accounting estimates [line items]              
IFRS 16 and 15 transition adjustments(1)       15 17    
Contract liabilities (Note 5)       15 $ 88 $ 62  
IFRS 16              
Disclosure of changes in accounting estimates [line items]              
Finance lease liabilities 63            
Expense relating to leases of low-value assets for which recognition exemption has been used       (1)      
Termination benefits expense       4      
Undiscounted operating lease payments to be received       $ 83      
Revisions in discount rates (31)            
Minimum | IFRS 16              
Disclosure of changes in accounting estimates [line items]              
Lease liabilities 42            
Maximum | IFRS 16              
Disclosure of changes in accounting estimates [line items]              
Lease liabilities $ 52