Employee retirement benefits - Components of Net Periodic Benefit Cost and Changes in Amounts Recorded in Accumulated Other Comprehensive Income (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Changes in amounts recorded in accumulated other comprehensive income
Total recorded in other comprehensive income	$ 218	$ (880)	$ (1,076)
Pension benefits
Components of net periodic benefit cost
Service cost	162	280	324
Interest cost	373	295	271
Expected return on plan assets	(373)	(412)	(427)
Amortization of prior service cost	19	17	17
Amortization of actuarial loss (gain)	44	84	143
Net periodic benefit cost	225	264	328
Changes in amounts recorded in accumulated other comprehensive income
Net actuarial loss (gain)	102	(522)	(817)
Amortization of net actuarial (loss) gain included in net periodic benefit cost	(44)	(84)	(143)
Prior service cost	184	0	0
Amortization of prior service cost included in net periodic benefit cost	(19)	(17)	(17)
Total recorded in other comprehensive income	223	(623)	(977)
Total recorded in net periodic benefit cost and other comprehensive income, before-tax	448	(359)	(649)
Other postretirement benefits
Components of net periodic benefit cost
Service cost	12	23	28
Interest cost	28	24	22
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	0
Amortization of actuarial loss (gain)	(9)	9	16
Net periodic benefit cost	31	56	66
Changes in amounts recorded in accumulated other comprehensive income
Net actuarial loss (gain)	(14)	(248)	(83)
Amortization of net actuarial (loss) gain included in net periodic benefit cost	9	(9)	(16)
Prior service cost	0	0	0
Amortization of prior service cost included in net periodic benefit cost	0	0	0
Total recorded in other comprehensive income	(5)	(257)	(99)
Total recorded in net periodic benefit cost and other comprehensive income, before-tax	$ 26	$ (201)	$ (33)