Financial Instruments (Details)	3 Months Ended						12 Months Ended
	Mar. 31, 2016 CAD	Mar. 31, 2016 USD ($)	Sep. 30, 2015 CAD	Sep. 30, 2015 USD ($)	Mar. 31, 2015 CAD	Mar. 31, 2015 USD ($)	Dec. 31, 2015 CAD	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Investments [Line Items]
Fair value	CAD 9,491,000,000				CAD 9,750,000,000
Loss on derivative			CAD 47,000,000	$ 36,000,000
Derivative losses amortized to net interest expense	5,000,000
Derivative ineffective portion	CAD 0	$ 0
Period to amortize AOCI	10 years	10 years
Gain on derivative	CAD 65,000,000						CAD 13,000,000
Other Comprehensive Income (Loss) [Member]
Schedule of Investments [Line Items]
Gain on derivative	63,000,000
Accounts Payable and Accrued Liabilities [Member]
Schedule of Investments [Line Items]
Loss on derivative	112,000,000						CAD 60,000,000
Net Interest Expense [Member]
Schedule of Investments [Line Items]
Loss on derivative					1,000,000
Loss amortized to net interest expense	1,000,000				1,000,000
Derivative losses amortized to net interest expense	6,000,000
Derivative ineffective portion	2,000,000				2,000,000	$ 2,000,000
Swap [Member]
Schedule of Investments [Line Items]
Loss on derivative					63,000,000	50,000,000
Effective portion of derivatives			CAD 47,000,000	36,000,000	CAD 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 | $				700,000,000				$ 700,000,000
Debt instrument face amount | $								$ 700,000,000
Interest rate					2.90%			2.90%
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
Debt instrument face amount | $				$ 900,000,000
Interest rate				6.125%
Forward Starting Interest Rate Swaps [Member]
Schedule of Investments [Line Items]
Floating-to-fixed interest rate swap agreements | $									$ 1,400,000,000
Net Investment Hedging [Member]
Schedule of Investments [Line Items]
Gain (loss) on effective portion of derivative instrument designated as hedge	308,000,000				CAD (356,000,000)
Ineffectiveness	0				0
Reported Value Measurement [Member]
Schedule of Investments [Line Items]
Carrying value	CAD 8,453,000,000				CAD 8,957,000,000