Financial Instruments (Details) CAD in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2017 CAD	Jun. 30, 2017 USD ($)	Jun. 30, 2016 CAD	Jun. 30, 2017 CAD	Jun. 30, 2016 CAD	Jun. 30, 2017 USD ($)	Dec. 31, 2016 CAD
Schedule of Investments [Line Items]
Fair value of long-term debt	CAD 9,925			CAD 9,925			CAD 9,981
Gain (loss) on effective portion of derivative instrument designated as hedge	(13)		CAD (32)	(11)	CAD (82)
Payment for settlement of forward starting swaps	22		0	22	0
Charge on hedge roll and de-designation	(13)		0	(13)	0
Gain (Loss) on Derivative, Net	(14)		(32)	(12)	(84)
Derivative gain (loss) amortized to net interest expense				(11)
Net Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative, ineffective portion loss	1		0	1	2
Amortized Net Interest Expensed [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense	2		3	5	5
Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Fair value loss	59			59			69
Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
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	22	$ 16
Charge on hedge roll and de-designation	13
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge	116		24	162	332
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 8,422			CAD 8,422			CAD 8,684