Debt - Components of Long-term Debt (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 02, 2021	Nov. 24, 2021
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 19,771	$ 20,264
Long-term debt and finance lease obligation, excluding perpetual debt	19,724	20,220
Unamortized fees on long-term debt	(120)	(137)
Long-term debt, including current debt	19,651	20,127
Long-term debt maturing within one year	1,510	1,550
Long-term debt	$ 18,141	18,577
5.100% 10-year Medium Term Notes
Debt Instrument [Line Items]
Interest rate	5.10%
Debt term	10 years
Long-term debt, including unamortized fees	$ 0	125
4.500% 10-year Notes
Debt Instrument [Line Items]
Interest rate	4.50%
Debt term	10 years
Long-term debt, including unamortized fees	$ 0	317
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	$ 474	444
1.589% 2-year Notes
Debt Instrument [Line Items]
Interest rate	1.589%			1.589%
Debt term	2 years
Long-term debt, including unamortized fees	$ 1,000	1,000
1.350% 3-year Notes
Debt Instrument [Line Items]
Interest rate	1.35%		1.35%
Debt term	3 years
Long-term debt, including unamortized fees	$ 2,030	1,899
2.900% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.90%
Debt term	10 years
Long-term debt, including unamortized fees	$ 948	887
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	$ 338	317
1.750% 5-year Notes
Debt Instrument [Line Items]
Interest rate	1.75%		1.75%
Debt term	5 years
Long-term debt, including unamortized fees	$ 1,353	1,266
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, 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	$ 677	634
3.150% 10-year Notes
Debt Instrument [Line Items]
Interest rate	3.15%
Debt term	10 years
Long-term debt, including unamortized fees	$ 399	399
2.050% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.05%
Debt term	10 years
Long-term debt, including unamortized fees	$ 676	633
7.125% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	7.125%
Debt term	30 years
Long-term debt, including unamortized fees	$ 474	444
2.450% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.45%		2.45%
Debt term	10 years
Long-term debt, including unamortized fees	$ 1,896	1,774
5.750% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	5.75%
Debt term	30 years
Long-term debt, including unamortized fees	$ 333	311
4.800% 20-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%
Debt term	20 years
Long-term debt, including unamortized fees	$ 405	379
5.950% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.95%
Debt term	30 years
Long-term debt, including unamortized fees	$ 603	564
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%		3.00%
Debt term	20 years
Long-term debt, including unamortized fees	$ 1,348	1,261
5.750% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.75%
Debt term	30 years
Long-term debt, including unamortized fees	$ 334	312
4.800% 30-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%
Debt term	30 years
Long-term debt, including unamortized fees	$ 743	695
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%		3.10%
Debt term	30 years
Long-term debt, including unamortized fees	$ 2,422	2,266
6.125% 100-year Notes
Debt Instrument [Line Items]
Interest rate	6.125%
Debt term	100 years
Long-term debt, including unamortized fees	$ 1,219	1,141
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Interest rate	5.41%
Long-term debt, including unamortized fees	$ 76	80
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Interest rate	6.91%
Long-term debt, including unamortized fees	$ 40	58
1.99-4.13% Finance Leases
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 2	2
6.99% Finance Lease
Debt Instrument [Line Items]
Interest rate	6.99%
Long-term debt, including unamortized fees	$ 0	97
6.57% Finance Lease
Debt Instrument [Line Items]
Interest rate	6.57%
Long-term debt, including unamortized fees	$ 29	33
12.77% Finance Lease
Debt Instrument [Line Items]
Interest rate	12.77%
Long-term debt, including unamortized fees	$ 3	4
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	0	336
Term Credit Facility
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	0	634
Demand Promissory Note
Debt Instrument [Line Items]
Commercial paper	$ 0	6
Perpetual 4% Consolidated Debenture Stock (USD)
Debt Instrument [Line Items]
Interest rate	4.00%
Long-term debt, including unamortized fees	$ 41	38
Perpetual 4% Consolidated Debenture Stock (GBP)
Debt Instrument [Line Items]
Interest rate	4.00%
Long-term debt, including unamortized fees	$ 6	$ 6