Components of Net Periodic Benefit Cost (Detail) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension benefits
Current service cost	$ 81	$ 76
Interest cost	68	77
Expected return on plan assets	(107)	(98)
Amortization of prior service cost	4	4
Amortization of actuarial loss (gain)	36	38
Net periodic benefit cost	82	97
Other postretirement benefits
Current service cost	7	6
Interest cost	6	6
Amortization of actuarial loss (gain)	4	3
Net periodic benefit cost	$ 17	$ 15