Financial Instruments (Detail) $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2017 CAD	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Investments [Line Items]
Fair value of long-term debt				CAD 9,680,000,000	CAD 9,981,000,000
Deposits in the form of short-term Investments				0	0
Net unamortized gain related to FX forward contracts				0	1,000,000
Payment for settlement of forward starting swaps				22,000,000	0	CAD 0
Charge on hedge roll and de-designation				(13,000,000)	0	0
Forward Starting Swaps [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of hedge				(7,000,000)	(12,000,000)
Floating-to-fixed interest rate swaps, Notional amount | $			$ 700				$ 500	$ 700
Derivative, Notional Amount De-designated | $		$ 700	$ 700
Derivative, Notional Amount Settled | $		200
Payment for settlement of forward starting swaps	CAD 22,000,000	$ 16
Charge on hedge roll and de-designation	CAD (13,000,000)
Gain (Loss) on Derivative, Net				(8,000,000)	(9,000,000)
Forward Starting Swaps [Member] | Interest Expense [Member]
Schedule of Investments [Line Items]
Ineffective portion gain (loss)				(1,000,000)	3,000,000
Derivative losses amortized to net interest expense				11,000,000	11,000,000
Derivative gains (losses) expected to be amortized to net interest expense				(12,000,000)
Forward Starting Swaps [Member] | Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Fair value loss				55,000,000	69,000,000
Treasury Rate Locks [Member]
Schedule of Investments [Line Items]
Unamortized losses, Treasury rate locks				20,000,000	21,000,000
Treasury Rate Locks [Member] | Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense				1,000,000	0	0
Derivative gains (losses) expected to be amortized to net interest expense				(1,000,000)
Accumulated Translation Adjustment [Member]
Schedule of Investments [Line Items]
Amount of net gain reclassified from AOCI				1,000,000	1,000,000
Net Investment Hedge [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of hedge				319,000,000	150,000,000	(757,000,000)
Amount of ineffectiveness on net investment hedge				0	0	CAD 0
Carrying Value Measurement [Member]
Schedule of Investments [Line Items]
Carrying value of long-term debt				CAD 8,159,000,000	CAD 8,684,000,000