Financial Instruments - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2019 CAD ($)	Mar. 31, 2019 CAD ($)	Jun. 30, 2018 CAD ($)	Jun. 30, 2019 CAD ($)	Jun. 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%	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		$ 24,000,000	$ 19,000,000
Gain on Derivative			12,000,000	$ 0	31,000,000
Forward Starting Swaps [Member] | Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	$ 3,000,000		2,000,000	5,000,000	5,000,000
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	$ 120,000,000		$ (122,000,000)	$ 240,000,000	$ (273,000,000)