Financial Instruments (Details) CAD in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017 CAD	Jun. 30, 2017 CAD	Jun. 30, 2017 USD ($)	Sep. 30, 2016 CAD	Sep. 30, 2017 CAD	Sep. 30, 2016 CAD	Sep. 30, 2017 USD ($)	Dec. 31, 2016 CAD
Schedule of Investments [Line Items]
Fair value of long-term debt	CAD 9,587				CAD 9,587			CAD 9,981
Payment for settlement of forward starting swaps	0			CAD 0	22	CAD 0
Charge on hedge roll and de-designation	0			0	13	0
Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	3			3	8	8
Derivative gains (losses) expected to be amortized to net interest expense					(12)
Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge	0			0	(11)	(82)
Floating-to-fixed interest rate swaps | $							$ 500
Forward starting swaps, de-designated | $			$ 700
Forward starting swaps, settled | $			200
Payment for settlement of forward starting swaps		CAD 22	$ 16
Charge on hedge roll and de-designation		CAD 13
Gain (Loss) on Derivative, Net	0			0	(12)	(84)
Forward Starting Interest Rate Swaps [Member] | Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative, ineffective portion loss	0			0	1	2
Forward Starting Interest Rate Swaps [Member] | Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Fair value loss	59				59			69
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge	180			(72)	342	260
Amount of ineffectiveness on net investment hedges	0			CAD 0	0	CAD 0
Reported Value Measurement [Member]
Schedule of Investments [Line Items]
Carrying value of long-term debt	CAD 8,133				CAD 8,133			CAD 8,684