Financial Instruments (Detail) $ in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2018 CAD ($)	Jun. 30, 2017 CAD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2016 CAD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Investments [Line Items]
Deposits in the form of short-term Investments				$ 0	$ 0
Unrealized gain on cash flow hedges and other				28	(6)	$ (12)
Charge on hedge roll and de-designation				0	(13)	0
4.00% 10-year Notes [Member]
Schedule of Investments [Line Items]
Debt Instrument, Face Amount								$ 500
Debt Instrument, Interest Rate	4.00%							4.00%
Debt Instrument, Term	10 years
Forward Starting Swaps [Member]
Schedule of Investments [Line Items]
Floating-to-fixed interest rate swaps, Notional amount			$ 500				$ 0	$ 500	$ 500
Fair value loss	$ 24							$ 19
Derivative, Notional Amount De-designated			700
Derivative, Notional Amount Settled			200
Payment for settlement of forward starting swaps		$ 22	$ 16
Charge on hedge roll and de-designation		$ (13)
Gain (Loss) on Derivative, Net				31	(8)
Forward Starting Swaps [Member] | Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense				10	11
Derivative losses expected to be amortized to net interest expense				(9)
Forward Starting Swaps [Member] | Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Fair value loss				0	55
Treasury Rate Locks [Member]
Schedule of Investments [Line Items]
Unamortized losses, Treasury rate locks				19	20
Treasury Rate Locks [Member] | Interest Expense [Member]
Schedule of Investments [Line Items]
Derivative losses amortized to net interest expense				1	1	0
Derivative losses expected to be amortized to net interest expense				(1)
Net Investment Hedge [Member]
Schedule of Investments [Line Items]
Unrealized gain on cash flow hedges and other				$ (479)	$ 319	$ 150