Debt - Components of Long-term Debt (Detail) CAD in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015 CAD	Dec. 31, 2015 USD ($)	Sep. 30, 2015	Dec. 31, 2014 CAD	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	CAD 9,025	$ 5,788		CAD 5,793	$ 4,047
Less: Unamortized fees on long-term debt	68			34
Long-term debt, including current debt	8,957			5,759
Less: Long-term debt maturing within one year	30			134
Long-term debt	CAD 8,927			CAD 5,625
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 380			CAD 319
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 374			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 484			CAD 405
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 346			CAD 290
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 343			CAD 287
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 483			CAD 405
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 484			CAD 406
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 339			CAD 282
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 615			CAD 515
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 340			CAD 284
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 968			CAD 0
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 345			CAD 0
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 759			CAD 0
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 413			CAD 0
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 1,246			CAD 0
Secured Equipment Loan [Member]
Debt Instrument [Line Items]
Maturity	2015-08
Currency in which payable	CDN$
Long-term debt, including unamortized fees	CAD 0			CAD 62
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 138			CAD 121
Secured Equipment Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.91%	6.91%
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 145			CAD 156
Senior Secured Notes Two [Member]
Debt Instrument [Line Items]
Interest rate			5.57%
5.57% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.57%	5.57%		5.57%	5.57%
Maturity	2024-12
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 0			CAD 65
Equipment Trust Certificate [Member]
Debt Instrument [Line Items]
Interest rate	7.49%	7.49%
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 64			CAD 96
3.88% Senior Secured Notes Series A & B [Member]
Debt Instrument [Line Items]
Interest rate	3.88%	3.88%		3.88%	3.88%
Maturity earliest	2026-10
Maturity latest	2026-12
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 0			CAD 148
4.28% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.28%	4.28%	4.28%	4.28%	4.28%
Maturity	2027-03
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 0			CAD 77
Other Long-term Loans (nil% - 5.50%) [Member]
Debt Instrument [Line Items]
Maturity earliest	2016-12
Maturity latest	2025-12
Currency in which payable	U.S.$ / CDN$
Long-term debt, including unamortized fees	CAD 10			CAD 2
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]
Maturity earliest	2022-12
Maturity latest	2026-12
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 172			CAD 147
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
Commercial Paper [Member]
Debt Instrument [Line Items]
Currency in which payable	U.S.$
Long-term debt, including unamortized fees	CAD 0			783
Total Long-term Debt Before Perpetual Consolidated Debenture Stock [Member]
Debt Instrument [Line Items]
Long-term debt, including unamortized fees	CAD 8,976			CAD 5,752
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 42			CAD 35
Perpetual 4% Consolidated Debenture Stock (GB) [Member]
Debt Instrument [Line Items]
Currency in which payable	G.B.£
Long-term debt, including unamortized fees	CAD 7			CAD 6