Debt - Components of Long-term Debt (Details) - CAD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 02, 2021	Nov. 24, 2021
Debt Instrument [Line Items]
Long-term debt, including unamortized fees		$ 20,264	$ 9,833
Long-term debt and finance lease obligation, excluding perpetual debt		20,220	9,788
Unamortized fees on long-term debt		(137)	(62)
Long-term debt, including current debt		20,127	9,771
Long-term debt maturing within one year		1,550	1,186
Long-term debt		$ 18,577	8,585
9.450% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	9.45%	9.45%
Debt term	30 years	30 years
Long-term debt, including unamortized fees		$ 0	318
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		$ 125	125
4.500% 10-year Notes
Debt Instrument [Line Items]
Interest rate		4.50%
Debt term		10 years
Long-term debt, including unamortized fees		$ 317	318
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		$ 444	445
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	0
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		$ 1,899	0
2.900% 10-year Notes
Debt Instrument [Line Items]
Interest rate		2.90%
Debt term		10 years
Long-term debt, including unamortized fees		$ 887	891
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		$ 317	318
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		$ 1,266	0
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	0
4.000% 10-year Notes
Debt Instrument [Line Items]
Interest rate		4.00%
Debt term		10 years
Long-term debt, including unamortized fees		$ 634	636
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%	2.05%
Debt term		10 years	10 years
Long-term debt, including unamortized fees		$ 633	$ 636
7.125% 30-year Debentures
Debt Instrument [Line Items]
Interest rate		7.125%
Debt term		30 years
Long-term debt, including unamortized fees		$ 444	446
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		$ 1,774	0
5.750% 30-year Debentures
Debt Instrument [Line Items]
Interest rate		5.75%
Debt term		30 years
Long-term debt, including unamortized fees		$ 311	312
4.800% 20-year Notes
Debt Instrument [Line Items]
Interest rate		4.80%
Debt term		20 years
Long-term debt, including unamortized fees		$ 379	381
5.950% 30-year Notes
Debt Instrument [Line Items]
Interest rate		5.95%
Debt term		30 years
Long-term debt, including unamortized fees		$ 564	567
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		$ 1,261	0
5.750% 30-year Notes
Debt Instrument [Line Items]
Interest rate		5.75%
Debt term		30 years
Long-term debt, including unamortized fees		$ 312	313
4.800% 30-year Notes
Debt Instrument [Line Items]
Interest rate		4.80%
Debt term		30 years
Long-term debt, including unamortized fees		$ 695	$ 698
3.050% 30-year Notes
Debt Instrument [Line Items]
Interest rate		3.05%	3.05%
Debt term		30 years	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		$ 2,266	0
6.125% 100-year Notes
Debt Instrument [Line Items]
Interest rate		6.125%
Debt term		100 years
Long-term debt, including unamortized fees		$ 1,141	1,146
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Interest rate		5.41%
Long-term debt, including unamortized fees		$ 80	89
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Interest rate		6.91%
Long-term debt, including unamortized fees		$ 58	75
7.49% Equipment Trust Certificates
Debt Instrument [Line Items]
Interest rate		7.49%
Long-term debt, including unamortized fees		$ 0	14
1.99-4.13% Finance Leases
Debt Instrument [Line Items]
Long-term debt, including unamortized fees		$ 2	4
1.99-4.13% Finance Leases | Minimum
Debt Instrument [Line Items]
Interest rate		1.99%
1.99-4.13% Finance Leases | Maximum
Debt Instrument [Line Items]
Interest rate		4.13%
6.99% Finance Lease
Debt Instrument [Line Items]
Interest rate		6.99%
Long-term debt, including unamortized fees		$ 97	97
6.57% Finance Lease
Debt Instrument [Line Items]
Interest rate		6.57%
Long-term debt, including unamortized fees		$ 33	38
12.77% Finance Lease
Debt Instrument [Line Items]
Interest rate		12.77%
Long-term debt, including unamortized fees		$ 4	4
1.93% Finance Lease
Debt Instrument [Line Items]
Interest rate		1.93%
Long-term debt, including unamortized fees		$ 4	0
Commercial Paper
Debt Instrument [Line Items]
Commercial paper		336	820
Term Credit Facility
Debt Instrument [Line Items]
Long-term debt, including unamortized fees		634	0
Demand Promissory Note
Debt Instrument [Line Items]
Commercial paper		$ 6	0
Perpetual 4% Consolidated Debenture Stock (USD)
Debt Instrument [Line Items]
Interest rate		4.00%
Long-term debt, including unamortized fees		$ 38	39
Perpetual 4% Consolidated Debenture Stock (GBP)
Debt Instrument [Line Items]
Interest rate		4.00%
Long-term debt, including unamortized fees		$ 6	$ 6