Components of Net Periodic Benefit Cost (Detail) - CAD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension benefits
Current service cost	$ 57	$ 59	$ 171	$ 179
Interest cost	81	76	243	227
Expected return on plan assets	(87)	(100)	(262)	(301)
Amortization of prior service cost	0	1	0	3
Amortization of actuarial loss (gain)	37	43	112	130
Net periodic benefit cost	88	79	264	238
Other postretirement benefits
Current service cost	4	5	12	13
Interest cost	5	5	16	16
Amortization of actuarial loss (gain)	0	2	(1)	5
Net periodic benefit cost	$ 9	$ 12	$ 27	$ 34