Debt - Components of Long-term Debt (Detail) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 8,757	$ 5,970	$ 8,219	$ 5,755
Less: Unamortized fees on long-term debt	61		60
Long-term debt, including current debt	8,696		8,159
Less: Long-term debt maturing within one year	506		746
Long-term debt	$ 8,190		$ 7,413
6.500% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.50%	6.50%	6.50%	6.50%
Debt Instrument, Maturity Date	May 15, 2018
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 0		$ 345
6.250% 10-year Medium Term Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.25%	6.25%	6.25%	6.25%
Debt Instrument, Maturity Date	Jun. 01, 2018
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 0		$ 375
7.250% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	7.25%	7.25%	7.25%	7.25%
Debt Instrument, Maturity Date	May 15, 2019
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 477		$ 439
9.450% 30-year Debentures [Member]
Debt Instrument [Line Items]
Interest rate	9.45%	9.45%	9.45%	9.45%
Debt Instrument, Maturity Date	Aug. 01, 2021
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 341		$ 314
5.100% 10-year Medium Term Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.10%	5.10%	5.10%	5.10%
Debt Instrument, Maturity Date	Jan. 14, 2022
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 125		$ 125
4.500% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.50%	4.50%	4.50%	4.50%
Debt Instrument, Maturity Date	Jan. 15, 2022
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 339		$ 311
4.450% 12.5-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.45%	4.45%	4.45%	4.45%
Debt Instrument, Maturity Date	Mar. 15, 2023
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 477		$ 438
2.900% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	2.90%	2.90%	2.90%	2.90%
Debt Instrument, Maturity Date	Feb. 01, 2025
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 955		$ 878
3.700% 10.5-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	3.70%	3.70%	3.70%	3.70%
Debt Instrument, Maturity Date	Feb. 02, 2026
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 340		$ 313
4.000% 10-year Notes [Member] [Domain]
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%
Debt Instrument, Maturity Date	Jun. 01, 2028
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 682		$ 0
7.125% 30-year Debentures [Member]
Debt Instrument [Line Items]
Interest rate	7.125%	7.125%	7.125%	7.125%
Debt Instrument, Maturity Date	Oct. 15, 2031
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 477		$ 439
5.750% 30-year Debentures [Member]
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%	5.75%	5.75%
Debt Instrument, Maturity Date	Mar. 15, 2033
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 334		$ 307
4.800% 20-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%	4.80%	4.80%
Debt Instrument, Maturity Date	Sep. 15, 2035
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 408		$ 375
5.950% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.95%	5.95%	5.95%	5.95%
Debt Instrument, Maturity Date	May 15, 2037
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 607		$ 558
6.450% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.45%	6.45%	6.45%	6.45%
Debt Instrument, Maturity Date	Nov. 17, 2039
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 400		$ 400
5.750% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%	5.75%	5.75%
Debt Instrument, Maturity Date	Jan. 15, 2042
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 336		$ 308
4.800% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%	4.80%	4.80%
Debt Instrument, Maturity Date	Aug. 01, 2045
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 748		$ 687
6.125% 100-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.125%	6.125%	6.125%	6.125%
Debt Instrument, Maturity Date	Sep. 15, 2115
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 1,228		$ 1,129
5.41% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.41%	5.41%	5.41%	5.41%
Debt Instrument, Maturity Date	Mar. 03, 2024
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 113		$ 111
6.91% Secured Equipment Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.91%	6.91%	6.91%	6.91%
Debt Instrument, Maturity Date	Oct. 01, 2024
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 106		$ 120
7.49% Equipment Trust Certificates [Member]
Debt Instrument [Line Items]
Interest rate	7.49%	7.49%	7.49%	7.49%
Debt Instrument, Maturity Date	Jan. 15, 2021
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 57		$ 52
Obligations Under Capital Leases (6.99%) [Member]
Debt Instrument [Line Items]
Interest rate	6.99%	6.99%	6.99%	6.99%
Debt Instrument, Maturity Date	Mar. 12, 2022
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 104		$ 96
Obligations Under Capital Leases (6.57%) [Member]
Debt Instrument [Line Items]
Interest rate	6.57%	6.57%	6.57%	6.57%
Debt Instrument, Maturity Date	Dec. 30, 2026
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 52		$ 52
Obligations Under Capital Leases (12.77%) [Member]
Debt Instrument [Line Items]
Interest rate	12.77%	12.77%	12.77%	12.77%
Debt Instrument, Maturity Date	Jan. 01, 2031
Currency in which payable	CDN$
Long-term debt, including unamortized fees	$ 4		$ 3
Total Long-term Debt Before Perpetual Consolidated Debenture Stock [Member]
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	$ 8,710		$ 8,175
Perpetual 4% Consolidated Debenture Stock (USD) [Member]
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%	4.00%	4.00%
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	$ 41		$ 38
Perpetual 4% Consolidated Debenture Stock (GBP) [Member]
Debt Instrument [Line Items]
Interest rate	4.00%	4.00%	4.00%	4.00%
Currency in which payable	G.B.£
Long-term debt, including unamortized fees	$ 6		$ 6