Components of Net Periodic Benefit Cost (Detail) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Current service cost	$ 59	$ 54	$ 179	$ 163
Interest cost	76	77	227	235
Expected return on plan assets	(100)	(104)	(301)	(306)
Amortization of prior service cost	1	2	3	7
Amortization of actuarial loss (gain)	43	43	130	132
Net periodic benefit cost	79	72	238	231
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Current service cost	5	4	13	12
Interest cost	5	6	16	18
Amortization of actuarial loss (gain)	2	2	5	6
Net periodic benefit cost	$ 12	$ 12	$ 34	$ 36