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Risk Management - Additional Information (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2020
CAD ($)
Dec. 31, 2019
CAD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2017
CAD ($)
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Total Debt Principal     $ 5,898 $ 4,998    
Fair Value of Risk Management Assets and Liabilities $ 53,000,000          
1% increase/decrease in interest rate on floating rate debt 1,000,000 $ 3,000,000        
Cash and cash equivalents 378,000,000 186,000,000     $ 781,000,000 $ 610,000,000
Available borrowing capacity 1,100,000,000          
Uncommitted demand facilities 1,100,000,000          
WRB [Member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Available borrowing capacity     55      
Uncommitted demand facilities     55      
Committed Credit Facility [Member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Cash and cash equivalents $ 378,000,000          
Top of Range [Member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Target debt to adjusted EBITDA ratio 2.0          
Available borrowing capacity $ 1,600,000,000          
Uncommitted demand facilities 1,600,000,000          
Amount of undrawn facilities for general purposes $ 600,000,000          
Interest rate risk [member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Changes in interest rate percentage 1.00%          
Liquidity risk [member] | Committed Credit Facility [Member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Available borrowing capacity $ 4,500,000,000          
Uncommitted demand facilities 4,500,000,000          
Liquidity risk [member] | Top of Range [Member] | Base Shelf Prospectus [Member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Available borrowing capacity     3,700      
Uncommitted demand facilities     $ 3,700      
Interest Rate, Foreign Exchange or Cross Currency Interest Rate Swap [Member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Total Debt Principal 0          
Notional Amount of Interest Rate Swaps [member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Total Debt Principal   $ 0        
Cross Currency Interest Rate Swap            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Total Debt Principal $ 0          
Trade receivables [member] | Credit risk [member] | Investment grade counterparties [member]            
Disclosure of Nature and Extent of Risks Arising from Financial Instruments [Line Items]            
Percent of accounts receivable held with investment grade counterparties 98.00% 97.00% 98.00% 97.00%    
Number of days outstanding for accruals, joint operations, trade receivables and net investment in finance leases 60 days 60 days        
Number of counterparties whose net settlement position individually accounts for more than 10 percent of the fair value of accruals, joint operations, trade receivables and net investment in finance lease 1 1 1 1    
Average expected credit loss rate 0.50% 0.30% 0.50% 0.30%    
Percentage of counterparties greater than 10 percent of accruals, joint operations, trade receivables and net investment in finance leases 10.00%