Financial Instruments (Detail)	3 Months Ended				12 Months Ended
	Sep. 30, 2015 CAD	Sep. 30, 2015 USD ($)	Mar. 31, 2015 CAD	Mar. 31, 2015 USD ($)	Dec. 31, 2015 CAD	Dec. 31, 2014 CAD	Dec. 31, 2013 CAD	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Investments [Line Items]
Fair Value of Long-term debt					CAD 9,750,000,000	CAD 6,939,000,000
Capital lease settlement						8,000,000
Loss on derivative	CAD 47,000,000	$ 36,000,000
Derivative losses amortized to net interest expense					CAD 5,000,000
Period to amortize AOCI					10 years
Gain on derivative	13,000,000
Combined realized and unrealized foreign exchange gain (loss )						3,000,000
Unrealized gain (loss) derived from foreign exchange forward contract					CAD (1,000,000)	CAD (3,000,000)	CAD (18,000,000)
6.500% 10-year Notes [Member]
Schedule of Investments [Line Items]
Interest rate					6.50%	6.50%		6.50%			6.50%
Maturity					2018-05
7.250% Notes due in May 2019 [Member]
Schedule of Investments [Line Items]
Interest rate						7.25%					7.25%
Maturity					2019-05
Capital Lease [Member]
Schedule of Investments [Line Items]
Capital lease settlement						CAD 17,000,000
Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Floating-to-fixed interest rate swap agreements | $								$ 1,400,000,000
Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Notional amount of interest rate swaps | $											$ 600,000,000
Treasury rate locks [Member]
Schedule of Investments [Line Items]
Unamortized losses related to interest rate locks accounted as cash flow hedges					CAD 21,000,000	21,000,000
Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Loss on derivative	60,000,000
Accumulated Translation Adjustment [Member]
Schedule of Investments [Line Items]
Reclassification from AOCI					1,000,000
Accumulated Other Comprehensive Income (Loss) [Member] | Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge						46,000,000
Accumulated Other Comprehensive Income (Loss) [Member] | Accounts Payable and Accrued Liabilities [Member] | Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge						21,000,000
Accumulated Other Comprehensive Income (Loss) [Member] | Other Long-Term Liabilities [Member] | Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge						25,000,000
Foreign Exchange Forward [Member] | Retained Earnings [Member]
Schedule of Investments [Line Items]
Unrealized gain (loss) derived from foreign exchange forward contract						(14,000,000)
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge					757,000,000	319,000,000	217,000,000
Amount of ineffectiveness on net investment hedges							CAD 0
Carrying Value Disclosure [Member]
Schedule of Investments [Line Items]
Long-term debt					CAD 8,957,000,000	CAD 5,759,000,000
Swap [Member]
Schedule of Investments [Line Items]
Loss on derivative			CAD 63,000,000	$ 50,000,000
Effective portion of derivatives	47,000,000	$ 36,000,000	60,000,000	48,000,000
2.900% 10-year Notes [Member] | Notes Payable to Banks [Member]
Schedule of Investments [Line Items]
Notional amount of interest rate swaps	CAD 700,000,000		CAD 700,000,000						$ 700,000,000	$ 700,000,000
Interest rate			2.90%							2.90%
Debt instrument face amount | $										$ 700,000,000
6.125% 100-year Notes [Member] | Notes Payable to Banks [Member]
Schedule of Investments [Line Items]
Notional amount of interest rate swaps | $									$ 700,000,000
Interest rate	6.125%				6.25%			6.25%	6.125%
Debt instrument face amount | $									$ 900,000,000
Net Interest Expense [Member]
Schedule of Investments [Line Items]
Loss on derivative	CAD 1,000,000
Derivative losses amortized to net interest expense					CAD 6,000,000
Derivative ineffective portion			CAD 2,000,000	$ 2,000,000
Loss amortized to net interest expense					CAD 5,000,000