Debt - Components of Long-term Debt (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Jun. 30, 2018
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 9,833	$ 8,816
Long-term debt and finance lease obligation, excluding perpetual debt	9,788	8,771
Unamortized fees on long-term debt	(62)	(59)
Long-term debt, including current debt	9,771	8,757
Long-term debt maturing within one year	1,186	599
Long-term debt	$ 8,585	$ 8,158
9.450% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	9.45%	9.45%
Maturity date	Aug. 01, 2021
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 318	$ 325
5.100% 10-year Medium Term Notes
Debt Instrument [Line Items]
Interest rate	5.10%	5.10%
Maturity date	Jan. 14, 2022
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 125	$ 125
4.500% 10-year Notes
Debt Instrument [Line Items]
Interest rate	4.50%	4.50%
Maturity date	Jan. 15, 2022
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 318	$ 324
4.450% 12.5-year Notes
Debt Instrument [Line Items]
Interest rate	4.45%	4.45%
Maturity date	Mar. 15, 2023
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 445	$ 454
2.900% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.90%	2.90%
Maturity date	Feb. 01, 2025
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 891	$ 909
3.700% 10.5-year Notes
Debt Instrument [Line Items]
Interest rate	3.70%	3.70%
Maturity date	Feb. 02, 2026
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 318	$ 324
4.000% 10-year Notes
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%	4.00%
Maturity date	Jun. 01, 2028
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 636	$ 649
3.150% 10-year Notes
Debt Instrument [Line Items]
Interest rate	3.15%	3.15%
Maturity date	Mar. 13, 2029	Mar. 13, 2029
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 399	$ 399
2.050% 10-year Notes
Debt Instrument [Line Items]
Interest rate	2.05%
Maturity date	Mar. 05, 2030
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 636	$ 0
7.125% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	7.125%	7.125%
Maturity date	Oct. 15, 2031
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 446	$ 454
5.750% 30-year Debentures
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%
Maturity date	Mar. 15, 2033
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 312	$ 318
4.800% 20-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%
Maturity date	Sep. 15, 2035
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 381	$ 388
5.950% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.95%	5.95%
Maturity date	May 15, 2037
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 567	$ 578
6.450% 30-year Notes
Debt Instrument [Line Items]
Interest rate	6.45%	6.45%
Maturity date	Nov. 17, 2039
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 400	$ 400
5.750% 30-year Notes
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%
Maturity date	Jan. 15, 2042
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 313	$ 319
4.800% 30-year Notes
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%
Maturity date	Aug. 01, 2045
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 698	$ 712
3.050% 30-year Notes
Debt Instrument [Line Items]
Interest rate	3.05%
Maturity date	Mar. 09, 2050
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 298	$ 0
6.125% 100-year Notes
Debt Instrument [Line Items]
Interest rate	6.125%	6.125%
Maturity date	Sep. 15, 2115
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 1,146	$ 1,169
8.000% 5-year Promissory Notes
Debt Instrument [Line Items]
Interest rate	8.00%	8.00%
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 0	$ 11
5.41% Senior Secured Notes
Debt Instrument [Line Items]
Interest rate	5.41%	5.41%
Maturity date	Mar. 03, 2024
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 89	$ 100
6.91% Secured Equipment Notes
Debt Instrument [Line Items]
Interest rate	6.91%	6.91%
Maturity date	Oct. 01, 2024
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 75	$ 91
7.49% Equipment Trust Certificates
Debt Instrument [Line Items]
Interest rate	7.49%	7.49%
Maturity date	Jan. 15, 2021
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 14	$ 55
1.99-2.97% Finance Leases
Debt Instrument [Line Items]
Currency in which payable	CDN$/U.S.$
Long-term debt, including unamortized fees	$ 4	$ 3
1.99-2.97% Finance Leases | Minimum
Debt Instrument [Line Items]
Interest rate	1.99%	1.99%
Maturity date	Jul. 01, 2021
1.99-2.97% Finance Leases | Maximum
Debt Instrument [Line Items]
Interest rate	2.97%	2.97%
Maturity date	Mar. 01, 2023
6.99% Finance Lease
Debt Instrument [Line Items]
Interest rate	6.99%	6.99%
Maturity date	Mar. 12, 2022
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 97	$ 99
6.57% Finance Lease
Debt Instrument [Line Items]
Interest rate	6.57%	6.57%
Maturity date	Dec. 30, 2026
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 38	$ 45
12.77% Finance Lease
Debt Instrument [Line Items]
Interest rate	12.77%	12.77%
Maturity date	Jan. 01, 2031
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 4	$ 4
Commercial Paper
Debt Instrument [Line Items]
Currency in which payable	U.S.$
Commercial paper	$ 820	$ 516
Commercial Paper | Maximum
Debt Instrument [Line Items]
Maturity date	Feb. 09, 2021
Perpetual 4% Consolidated Debenture Stock (USD)
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 39	$ 39
Perpetual 4% Consolidated Debenture Stock (GBP)
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Currency in which payable	G.B.£
Long-term debt, including unamortized fees	$ 6	$ 6