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Financial Instruments (Details) $ in Millions, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018 CAD ($)	Jun. 30, 2018 CAD ($)	Sep. 30, 2017 CAD ($)	Sep. 30, 2018 CAD ($)	Sep. 30, 2017 CAD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
U.S. $500 million 4.000% 10-year Notes
Schedule of Investments [Line Items]
Debt Instrument, Face Amount						$ 500
Debt Instrument, Interest Rate						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	$ 500
Fair value loss		$ (24)					$ (19)
Gain (Loss) on Derivative, Net				$ 31	$ (12)
Forward Starting Swaps [Member] \| Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	$ 2		$ 3	7	8
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	$ 96		$ 180	$ (177)	$ 342