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Financial Instruments (Details) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 CAD ($)	Jun. 30, 2018 CAD ($)	Mar. 31, 2018 CAD ($)	Jun. 30, 2018 USD ($)
U.S. $500 million 4.000% 10-year Notes
Schedule of Investments [Line Items]
Debt Instrument, Face Amount	$ 400			$ 500
Debt Instrument, Interest Rate	3.15%	4.00%		4.00%
Debt Instrument, Term	10 years	10 years
Forward Starting Swaps [Member]
Schedule of Investments [Line Items]
Floating-to-fixed interest rate swaps, Notional amount				$ 500
Fair value loss		$ 24		$ 19
Derivative, Gain on Derivative			$ 19
Forward Starting Swaps [Member] \| Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	$ 2		3
Derivative losses expected to be amortized to net interest expense	9
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative, Net	$ 120		$ (151)