Debt - Components of Long-term Debt (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2018
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 8,816	$ 8,757
Long-term debt and finance lease obligation, excluding perpetual debt	8,771	8,710
Unamortized fees on long-term debt	(59)	(61)
Long-term debt, including current debt	8,757	8,696
Long-term debt maturing within one year	599	506
Long-term debt	$ 8,158	$ 8,190
7.250% 10-year Notes
Debt Instrument [Line Items]
Interest rate	7.25%	7.25%
Maturity date	May 15, 2019
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 0	$ 477
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	$ 325	$ 341
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	$ 324	$ 339
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	$ 454	$ 477
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	$ 909	$ 955
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	$ 324	$ 340
4.000% 10-year Notes
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%	4.00%
Maturity date	Jun. 01, 2028	Jun. 01, 2028
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 649	$ 682
3.150% 10-year Notes
Debt Instrument [Line Items]
Interest rate	3.15%
Maturity date	Mar. 13, 2029
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 399	$ 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	$ 454	$ 477
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	$ 318	$ 334
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	$ 388	$ 408
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	$ 578	$ 607
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	$ 319	$ 336
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	$ 712	$ 748
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	$ 1,169	$ 1,228
8.000% 5-year Promissory Notes
Debt Instrument [Line Items]
Interest rate	8.00%
Maturity date	Jun. 30, 2020
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 11	$ 0
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	$ 100	$ 113
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	$ 91	$ 106
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	$ 55	$ 57
2.97% Finance Lease
Debt Instrument [Line Items]
Interest rate	2.97%	2.97%
Maturity date	Jun. 01, 2020
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 3	$ 0
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	$ 99	$ 104
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	$ 45	$ 52
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	$ 516	$ 0
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	$ 41
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