Debt - Components of Long-term Debt (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 22,598	$ 19,771
Long-term debt and finance lease obligation, excluding perpetual debt	22,552	19,724
Unamortized fees on long-term debt	(104)	(120)
Long-term debt, including current debt	22,494	19,651
Long-term debt maturing within one year (Note 17, 18, 20)	3,143	1,510
Long-term debt	$ 19,351	18,141
4.450% 12.5-year Notes
Debt Instrument [Line Items]
Interest rate	4.45%
Debt term	12 years 6 months
Long-term debt, including unamortized fees	$ 0	474
1.589% 2-year Notes
Debt Instrument [Line Items]
Interest rate	1.589%
Debt term	2 years
Long-term debt, including unamortized fees	$ 0	1,000
1.350% 3-year Notes
Debt Instrument [Line Items]
Interest rate	1.35%
Debt term	3 years
Long-term debt, including unamortized fees	$ 1,983	2,030
2.900% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.90%
Debt term	10 years
Long-term debt, including unamortized fees	$ 926	948
3.700% 10.5-year Notes
Debt Instrument [Line Items]
Interest rate	3.70%
Debt term	10 years 6 months
Long-term debt, including unamortized fees	$ 330	338
1.750% 5-year Notes
Debt Instrument [Line Items]
Interest rate	1.75%
Debt term	5 years
Long-term debt, including unamortized fees	$ 1,321	1,353
2.540% 6.3-year Notes
Debt Instrument [Line Items]
Interest rate	2.54%
Debt term	6 years 3 months 18 days
Long-term debt, including unamortized fees	$ 1,200	1,200
4.000% 10-year Notes
Debt Instrument [Line Items]
Interest rate	4.00%
Debt term	10 years
Long-term debt, including unamortized fees	$ 661	677
3.150% 10-year Notes
Debt Instrument [Line Items]
Interest rate	3.15%
Debt term	10 years
Long-term debt, including unamortized fees	$ 400	399
2.050% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.05%
Debt term	10 years
Long-term debt, including unamortized fees	$ 660	676
7.125% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	7.125%
Debt term	30 years
Long-term debt, including unamortized fees	$ 463	474
2.450% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.45%
Debt term	10 years
Long-term debt, including unamortized fees	$ 1,851	1,896
5.750% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	5.75%
Debt term	30 years
Long-term debt, including unamortized fees	$ 326	333
4.800% 20-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%
Debt term	20 years
Long-term debt, including unamortized fees	$ 396	405
5.950% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.95%
Debt term	30 years
Long-term debt, including unamortized fees	$ 590	603
6.450% 30-year Notes
Debt Instrument [Line Items]
Interest rate	6.45%
Debt term	30 years
Long-term debt, including unamortized fees	$ 400	400
3.000% 20-year Notes
Debt Instrument [Line Items]
Interest rate	3.00%
Debt term	20 years
Long-term debt, including unamortized fees	$ 1,317	1,348
5.750% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.75%
Debt term	30 years
Long-term debt, including unamortized fees	$ 326	334
4.800% 30-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%
Debt term	30 years
Long-term debt, including unamortized fees	$ 725	743
3.050% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.05%
Debt term	30 years
Long-term debt, including unamortized fees	$ 298	298
3.100% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.10%
Debt term	30 years
Long-term debt, including unamortized fees	$ 2,365	2,422
6.125% 100-year Notes
Debt Instrument [Line Items]
Interest rate	6.125%
Debt term	100 years
Long-term debt, including unamortized fees	$ 1,190	1,219
3.125% 10.1-year Notes
Debt Instrument [Line Items]
Interest rate	3.125%
Debt term	10 years
Long-term debt, including unamortized fees	$ 291	0
2.875% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.875%
Debt term	10 years
Long-term debt, including unamortized fees	$ 499	0
4.300% 30.1-year Notes
Debt Instrument [Line Items]
Interest rate	4.30%
Debt term	30 years
Long-term debt, including unamortized fees	$ 515	0
4.950% 30.1-year Notes
Debt Instrument [Line Items]
Interest rate	4.95%
Debt term	30 years
Long-term debt, including unamortized fees	$ 574	0
4.700% 30-year Notes
Debt Instrument [Line Items]
Interest rate	4.70%
Debt term	30 years
Long-term debt, including unamortized fees	$ 599	0
3.500% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.50%
Debt term	30 years
Long-term debt, including unamortized fees	$ 540	0
4.200% 50-year Notes
Debt Instrument [Line Items]
Interest rate	4.20%
Debt term	50 years
Long-term debt, including unamortized fees	$ 444	0
2.875% - 7.000% Other Senior Notes Due 2069
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 104	0
2.875% - 7.000% Other Senior Notes Due 2069 | Minimum
Debt Instrument [Line Items]
Interest rate	2.875%
2.875% - 7.000% Other Senior Notes Due 2069 | Maximum
Debt Instrument [Line Items]
Interest rate	7.00%
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Interest rate	5.41%
Long-term debt, including unamortized fees	$ 64	76
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Interest rate	6.91%
Long-term debt, including unamortized fees	$ 21	40
2.96% - 4.29% RRIF Loans
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 70	0
2.96% - 4.29% RRIF Loans | Minimum
Debt Instrument [Line Items]
Interest rate	2.96%
2.96% - 4.29% RRIF Loans | Maximum
Debt Instrument [Line Items]
Interest rate	4.29%
1.99-4.13% Finance Leases
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 8	2
2.32% Finance Lease
Debt Instrument [Line Items]
Interest rate	2.32%
Long-term debt, including unamortized fees	$ 8	0
6.57% Finance Lease
Debt Instrument [Line Items]
Interest rate	6.57%
Long-term debt, including unamortized fees	$ 22	29
12.77% Finance Lease
Debt Instrument [Line Items]
Interest rate	12.77%
Long-term debt, including unamortized fees	$ 3	3
1.93% Finance Lease
Debt Instrument [Line Items]
Interest rate	1.93%
Long-term debt, including unamortized fees	$ 4	4
Commercial Paper
Debt Instrument [Line Items]
Commercial Paper	$ 1,058	0
Perpetual 4% Consolidated Debenture Stock (USD)
Debt Instrument [Line Items]
Interest rate	4.00%
Long-term debt, including unamortized fees	$ 40	41
Perpetual 4% Consolidated Debenture Stock (GBP)
Debt Instrument [Line Items]
Interest rate	4.00%
Long-term debt, including unamortized fees	$ 6	$ 6