Financial Instruments - Narrative (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2019 CAD ($)	Mar. 31, 2019 CAD ($)	Sep. 30, 2018 CAD ($)	Jun. 30, 2018 CAD ($)	Sep. 30, 2019 CAD ($)	Sep. 30, 2018 CAD ($)	Jun. 30, 2018 USD ($)
U.S. $500 million 4.000% 10-year Notes
Schedule of Investments [Line Items]
Debt instrument, Face amount		$ 400,000,000					$ 500,000,000
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							$ 500,000,000
Fair value loss				$ 24,000,000			$ 19,000,000
Gain on Derivative			$ 0		$ 0	$ 31,000,000
Forward Starting Swaps [Member] | Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	$ 2,000,000		2,000,000		7,000,000	7,000,000
Derivative losses expected to be amortized to net interest expense					9,000,000
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative, Net	$ (68,000,000)		$ 96,000,000		$ 172,000,000	$ (177,000,000)