Financial Instruments - Narrative (Details) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021 CAD ($)	Mar. 31, 2020 CAD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 CAD ($)
Schedule of Investments [Line Items]
Debt and finance lease liabilities, Carrying value	$ 8,841				$ 8,951
Debt and finance lease liabilities, Fair value	10,609				$ 11,597
FX Forward Contracts
Schedule of Investments [Line Items]
Derivative, Notional amount				$ 800
FX Forward Contracts | Subsequent Event
Schedule of Investments [Line Items]
Derivative, Notional amount			$ 200
Forward Starting Swaps
Schedule of Investments [Line Items]
Derivative, Notional amount				$ 1,800
Unrealized gain (loss) on cash flow hedges	20
Forward Starting Swaps | Subsequent Event
Schedule of Investments [Line Items]
Derivative, Notional amount			$ 600
Bond Locks
Schedule of Investments [Line Items]
Derivative, Notional amount	600
Unrealized gain (loss) on cash flow hedges	2
Net Investment Hedge
Schedule of Investments [Line Items]
Gain (loss) on net investment hedge	$ 76	$ (555)