Components of Net Periodic Benefit Cost (Detail) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension benefits
Current service cost	$ 77	$ 57	$ 153	$ 114
Interest cost	77	81	154	162
Expected return on plan assets	(98)	(88)	(196)	(175)
Amortization of prior service cost	3	0	7	0
Amortization of actuarial loss (gain)	39	38	77	75
Net periodic benefit cost	98	88	195	176
Other postretirement benefits
Current service cost	6	4	12	8
Interest cost	6	6	12	11
Amortization of actuarial loss (gain)	3	(1)	6	(1)
Net periodic benefit cost	$ 15	$ 9	$ 30	$ 18