Debt - Components of Long-term Debt (Detail) CAD in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	CAD 8,749	CAD 9,025	$ 5,763	$ 5,788
Less: Unamortized fees on long-term debt	65	68
Long-term debt, including current debt	8,684	8,957
Less: Long-term debt maturing within one year	25	30
Long-term debt	CAD 8,659	CAD 8,927
6.500% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.50%	6.50%	6.50%	6.50%
Maturity	2018-05
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 369	CAD 380
6.250% 10-year Medium-term Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.25%	6.25%	6.25%	6.25%
Maturity	2018-06
Currency in which payable	CDN$
Long-term debt, including unamortized fees	CAD 375	CAD 374
7.250% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	7.25%	7.25%	7.25%	7.25%
Maturity	2019-05
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 469	CAD 484
9.450% 30-year Debentures [Member]
Debt Instrument [Line Items]
Interest rate	9.45%	9.45%	9.45%	9.45%
Maturity	2021-08
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 336	CAD 346
5.100% 10-year Medium Term Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.10%	5.10%	5.10%	5.10%
Maturity	2022-01
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%
Maturity	2022-01
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 333	CAD 343
4.450% 12.5 year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.45%	4.45%	4.45%	4.45%
Maturity	2023-03
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 469	CAD 483
7.125% 30-year Debentures [Member]
Debt Instrument [Line Items]
Interest rate	7.125%	7.125%	7.125%	7.125%
Maturity	2031-10
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 470	CAD 484
5.750% 30-year Debentures [Member]
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%	5.75%	5.75%
Maturity	2033-03
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 328	CAD 339
5.950% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.95%	5.95%	5.95%	5.95%
Maturity	2037-05
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 597	CAD 615
6.450% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.45%	6.45%	6.45%	6.45%
Maturity	2039-11
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%
Maturity	2042-01
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 330	CAD 340
2.900% 10-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	2.90%	2.90%	2.90%	2.90%
Maturity	2025-02
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 940	CAD 968
3.700% 10.5-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	3.70%	3.70%	3.70%	3.70%
Maturity	2026-02
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 335	CAD 345
4.800% 30-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%	4.80%	4.80%
Maturity	2045-08
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 736	CAD 759
4.800% 20-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.80%	4.80%	4.80%	4.80%
Maturity	2035-09
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 401	CAD 413
6.125% 100-year Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.125%	6.125%	6.125%	6.125%
Maturity	2115-09
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 1,208	CAD 1,246
5.41% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.41%	5.41%	5.41%	5.41%
Maturity	2024-03
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 126	CAD 138
6.91% Secured Equipment Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.91%	6.91%	6.91%	6.91%
Maturity	2024-10
Currency in which payable	CDN$
Long-term debt, including unamortized fees	CAD 133	CAD 145
7.49% Equipment Trust Certificates [Member]
Debt Instrument [Line Items]
Interest rate	7.49%	7.49%	7.49%	7.49%
Maturity	2021-01
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 56	CAD 64
Other Long-term Loans (nil% - 5.50%) [Member]
Debt Instrument [Line Items]
Currency in which payable	U.S.$/CDN$
Long-term debt, including unamortized fees	CAD 0	CAD 10
Debt Instrument, Maturity, Month and Year, Range, Start		2016-12
Debt Instrument, Maturity, Month and Year, Range, End	2025-12
Other Long-term Loans (nil% - 5.50%) [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate	0.00%	0.00%	0.00%	0.00%
Other Long-term Loans (nil% - 5.50%) [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate	5.50%	5.50%	5.50%	5.50%
Obligations Under Capital Leases (6.313% - 6.99%) [Member]
Debt Instrument [Line Items]
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 163	CAD 172
Debt Instrument, Maturity, Month and Year, Range, Start	2022-12
Debt Instrument, Maturity, Month and Year, Range, End	2026-12
Obligations Under Capital Leases (6.313% - 6.99%) [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate	6.313%	6.313%	6.313%	6.313%
Obligations Under Capital Leases (6.313% - 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%
Maturity		2031-01
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 8,702	CAD 8,976
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 41	CAD 42
Perpetual 4% Consolidated Debenture Stock (GB) [Member]
Debt Instrument [Line Items]
Currency in which payable	G.B.£
Long-term debt, including unamortized fees	CAD 6	CAD 7