Financial Instruments (Details) $ in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2018 CAD ($)	Jun. 30, 2017 CAD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)
Schedule of Investments [Line Items]
Charge on hedge roll and de-designation				$ 0	$ 0	$ 13
4.000% 10-year Notes
Schedule of Investments [Line Items]
Debt instrument, Face amount							$ 500	$ 500
Interest rate	4.00%			4.00%	4.00%		4.00%	4.00%
Debt instrument, Term	10 years				10 years
Forward Starting Swaps
Schedule of Investments [Line Items]
Gain (Loss) on Derivative, Net					$ 31
Floating-to-fixed interest rate swaps, Notional amount			$ 500				$ 500	$ 500
Fair value loss	$ 24							$ 19
Derivative, Notional Amount De-designated			700
Derivative, Notional Amount Settled			200
Payment for settlement of forward starting swaps		$ 22	$ 16
Charge on hedge roll and de-designation		$ 13
Forward Starting Swaps | Net Interest Expense
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense				$ 9	10	11
Derivative losses expected to be amortized to net interest expense				9
Treasury Rate Locks
Schedule of Investments [Line Items]
Unamortized losses, Treasury rate locks				18	19
Treasury Rate Locks | Net Interest Expense
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense				1	1	1
Derivative losses expected to be amortized to net interest expense				1
Net Investment Hedge
Schedule of Investments [Line Items]
Gain (Loss) on Derivative, Net				$ 288	$ (479)	$ 319