Debt - Components of Long-term Debt (Detail) CAD in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 CAD	Dec. 31, 2017 USD ($)	Dec. 31, 2016 CAD	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	CAD 8,219	$ 5,755	CAD 8,749	$ 5,763
Less: Unamortized fees on long-term debt	60		65
Long-term debt, including current debt	8,159		8,684
Less: Long-term debt maturing within one year	746		25
Long-term debt	CAD 7,413		CAD 8,659
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	CAD 345		CAD 369
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	CAD 375		CAD 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	CAD 439		CAD 469
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	CAD 314		CAD 336
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	CAD 125		CAD 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	CAD 311		CAD 333
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	CAD 438		CAD 469
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	CAD 878		CAD 940
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	CAD 313		CAD 335
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	CAD 439		CAD 470
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	CAD 307		CAD 328
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	CAD 375		CAD 401
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	CAD 558		CAD 597
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	CAD 400		CAD 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	CAD 308		CAD 330
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	CAD 687		CAD 736
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	CAD 1,129		CAD 1,208
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	CAD 111		CAD 126
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	CAD 120		CAD 133
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	CAD 52		CAD 56
Obligations Under Capital Leases (6.57% - 6.99%) [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date Range, Start	Mar. 12, 2022
Debt Instrument, Maturity Date Range, End	Dec. 30, 2026
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 148		CAD 163
Obligations Under Capital Leases (6.57% - 6.99%) [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate	6.57%	6.57%	6.57%	6.57%
Obligations Under Capital Leases (6.57% - 6.99%) [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate	6.99%	6.99%	6.99%	6.99%
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	CAD 3		CAD 3
Total Long-term Debt Before Perpetual Consolidated Debenture Stock [Member]
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	CAD 8,175		CAD 8,702
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	CAD 38		CAD 41
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	CAD 6		CAD 6