Financial Instruments (Details) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 CAD ($)	Mar. 31, 2017 CAD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)
Forward Starting Swaps [Member]
Schedule of Investments [Line Items]
Floating-to-fixed interest rate swaps, Notional amount			$ 500		$ 500
Gain (Loss) on Derivative, Net	$ 19	$ 2
Forward Starting Swaps [Member] | Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	3	3
Derivative gains (losses) expected to be amortized to net interest expense	(11)
Forward Starting Swaps [Member] | Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Fair value loss	(36)			$ (55)
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative, Net	$ (151)	$ 46