Financial Instruments - Narrative (Details) $ in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 CAD ($)	Jun. 30, 2020 CAD ($)	Jun. 30, 2021 CAD ($)	Jun. 30, 2020 CAD ($)	Jun. 30, 2021 USD ($)	May 21, 2021 CAD ($)	Dec. 31, 2020 CAD ($)
Schedule of Investments [Line Items]
Debt and finance lease liabilities, Carrying value	$ 8,722		$ 8,722				$ 8,951
Debt and finance lease liabilities, Fair value	10,756		10,756				$ 11,597
FX Forward Contracts
Schedule of Investments [Line Items]
Derivative, Notional amount					$ 1,000
Unrealized gain (loss) on derivatives	(17)		(17)
Forward Starting Swaps
Schedule of Investments [Line Items]
Derivative, Notional amount					$ 2,400
Gain (Loss) on Derivative Instruments, Net, Pretax	(240)		(220)
Forward Starting Swaps | Accounts Payable and Accrued Liabilities
Schedule of Investments [Line Items]
Derivative, Fair Value, Net	(220)		(220)
Forward Starting Swaps | Accumulated other comprehensive loss
Schedule of Investments [Line Items]
Cash Flow Hedges Derivative Instruments at Fair Value, Net						$ (73)
Forward Starting Swaps | Other expense (income)
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax			(147)
Forward Starting Swaps | Other Comprehensive Income
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax			(73)
Bond Locks
Schedule of Investments [Line Items]
Derivative, Notional amount	600		600
Gain (Loss) on Derivative Instruments, Net, Pretax	(7)		(5)
Bond Locks | Accounts Payable and Accrued Liabilities
Schedule of Investments [Line Items]
Derivative, Fair Value, Net	(5)		(5)
Bond Locks | Accumulated other comprehensive loss
Schedule of Investments [Line Items]
Cash Flow Hedges Derivative Instruments at Fair Value, Net						$ (2)
Bond Locks | Other expense (income)
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax			(3)
Bond Locks | Other Comprehensive Income
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax			(2)
Net Investment Hedge
Schedule of Investments [Line Items]
Gain (loss) on net investment hedge	$ 86	$ 264	$ 162	$ (291)