Debt - Components of Long-term Debt (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2024 CAD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 USD ($)
Debt Instrument [Line Items]
Long-term debt, gross		$ 14,598		$ 15,764
Obligations under finance leases	$ 21		$ 31
Commercial Paper	1,586		1,058
Short-term Borrowing	288		0
Debt, long-term, short-term and finance lease obligation, excluding perpetual debt	22,663		22,552
Debt, long-term, short-term and finance lease obligation	22,713		22,598
Unamortized fees on long-term debt	(90)		(104)
Long-term debt, including current debt	22,623		22,494
Less: Long-term debt maturing within one year	2,819		3,143
Long-term debt	$ 19,804		19,351
1.350% 3-year Notes
Debt Instrument [Line Items]
Interest rate	1.35%	1.35%
Debt term	3 years
Long-term debt, gross	$ 0		1,983
2.900% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.90%	2.90%
Debt term	10 years
Long-term debt, gross	$ 924		926
3.700% 10.5-year Notes
Debt Instrument [Line Items]
Interest rate	3.70%	3.70%
Debt term	10 years 6 months
Long-term debt, gross	$ 360		330
1.750% 5-year Notes
Debt Instrument [Line Items]
Interest rate	1.75%	1.75%
Debt term	5 years
Long-term debt, gross	$ 1,438		1,321
2.540% 6.3-year Notes
Debt Instrument [Line Items]
Interest rate	2.54%	2.54%
Debt term	6 years 3 months 18 days
Long-term debt, gross	$ 1,200		1,200
4.000% 10-year Notes
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Debt term	10 years
Long-term debt, gross	$ 719		661
3.150% 10-year Notes
Debt Instrument [Line Items]
Interest rate	3.15%	3.15%
Debt term	10 years
Long-term debt, gross	$ 400		400
2.050% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.05%	2.05%
Debt term	10 years
Long-term debt, gross	$ 719		660
7.125% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	7.125%	7.125%
Debt term	30 years
Long-term debt, gross	$ 503		463
2.450% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.45%	2.45%
Debt term	10 years
Long-term debt, gross	$ 2,014		1,851
5.750% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%
Debt term	30 years
Long-term debt, gross	$ 355		326
4.800% 20-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%
Debt term	20 years
Long-term debt, gross	$ 431		396
5.950% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.95%	5.95%
Debt term	30 years
Long-term debt, gross	$ 642		590
6.450% 30-year Notes
Debt Instrument [Line Items]
Interest rate	6.45%	6.45%
Debt term	30 years
Long-term debt, gross	$ 400		400
3.000% 20-year Notes
Debt Instrument [Line Items]
Interest rate	3.00%	3.00%
Debt term	20 years
Long-term debt, gross	$ 1,433		1,317
5.750% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%
Debt term	30 years
Long-term debt, gross	$ 355		326
4.800% 30-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%
Debt term	30 years
Long-term debt, gross	$ 790		725
3.050% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.05%	3.05%
Debt term	30 years
Long-term debt, gross	$ 298		298
3.100% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.10%	3.10%
Debt term	30 years
Long-term debt, gross	$ 2,507		2,365
6.125% 100-year Notes
Debt Instrument [Line Items]
Interest rate	6.125%	6.125%
Debt term	100 years
Long-term debt, gross	$ 1,295		1,190
3.125% 10.1-year Notes
Debt Instrument [Line Items]
Interest rate	3.125%	3.125%
Debt term	10 years
Long-term debt, gross	$ 320		291
2.875% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.875%	2.875%
Debt term	10 years
Long-term debt, gross	$ 551		499
4.300% 30.1-year Notes
Debt Instrument [Line Items]
Interest rate	4.30%	4.30%
Debt term	30 years
Long-term debt, gross	$ 563		515
4.950% 30.1-year Notes
Debt Instrument [Line Items]
Interest rate	4.95%	4.95%
Debt term	30 years
Long-term debt, gross	$ 626		574
4.700% 30-year Notes
Debt Instrument [Line Items]
Interest rate	4.70%	4.70%
Debt term	30 years
Long-term debt, gross	$ 653		599
3.500% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.50%	3.50%
Debt term	30 years
Long-term debt, gross	$ 591		540
4.200% 50-year Notes
Debt Instrument [Line Items]
Interest rate	4.20%	4.20%
Debt term	50 years
Long-term debt, gross	$ 484		444
2.875% - 7.000% Other Senior Notes Due 2069
Debt Instrument [Line Items]
Long-term debt, gross	$ 114		104
2.875% - 7.000% Other Senior Notes Due 2069 | Minimum
Debt Instrument [Line Items]
Interest rate	2.875%	2.875%
2.875% - 7.000% Other Senior Notes Due 2069 | Maximum
Debt Instrument [Line Items]
Interest rate	7.00%	7.00%
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Interest rate	5.41%	5.41%
Long-term debt, gross	$ 0		64
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Interest rate	6.91%	6.91%
Long-term debt, gross	$ 0		21
2.96% - 4.29% RRIF Loans
Debt Instrument [Line Items]
Long-term debt, gross	$ 69		70
2.96% - 4.29% RRIF Loans | Minimum
Debt Instrument [Line Items]
Interest rate	2.96%	2.96%
2.96% - 4.29% RRIF Loans | Maximum
Debt Instrument [Line Items]
Interest rate	4.29%	4.29%
1.99-4.13% Finance Leases
Debt Instrument [Line Items]
Obligations under finance leases	$ 6		8
2.32% Finance Lease
Debt Instrument [Line Items]
Interest rate	2.32%	2.32%
Obligations under finance leases	$ 6		8
6.57% Finance Lease
Debt Instrument [Line Items]
Interest rate	6.57%	6.57%
Obligations under finance leases	$ 16		22
12.77% Finance Lease
Debt Instrument [Line Items]
Interest rate	12.77%	12.77%
Obligations under finance leases	$ 3		3
1.93% Finance Lease
Debt Instrument [Line Items]
Interest rate	1.93%	1.93%
Obligations under finance leases	$ 4		4
Perpetual 4% Consolidated Debenture Stock (USD)
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Long-term debt, gross	$ 44		40
Perpetual 4% Consolidated Debenture Stock (GBP)
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Long-term debt, gross	$ 6		$ 6