Debt - Components of Long-term Debt - A (Narrative) (Details) CAD in Millions	3 Months Ended
	Sep. 30, 2015 CAD	Sep. 30, 2015 USD ($)	Mar. 31, 2015 CAD	Mar. 31, 2015 USD ($)
Notes Payable to Banks [Member] | 2.900% 10-year Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount				$ 700,000,000
Interest rate				2.90%
Debt instrument term			10 years	10 years
Proceeds from issuance of debt instrument			CAD 873	$ 694,000,000
Notional amount		$ 700,000,000		$ 700,000,000
Notes Payable to Banks [Member] | 4.800% 30-year Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 550,000,000
Interest rate		4.80%
Debt instrument term	30 years	30 years
Notes Payable to Banks [Member] | 3.700% 10.5-year Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 250,000,000
Interest rate		3.70%
Debt instrument term	10 years 6 months	10 years 6 months
Notes Payable to Banks [Member] | 4.800% and 3.700% Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 800,000,000
Proceeds from issuance of debt instrument	CAD 1,032	789,000,000
Notes Payable to Banks [Member] | 6.125% 100-year Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 900,000,000
Interest rate		6.125%
Debt instrument term	100 years	100 years
Notes Payable to Banks [Member] | 4.800% 20-year Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 300,000,000
Interest rate		4.80%
Debt instrument term	20 years	20 years
Notes Payable to Banks [Member] | 6.25% and 4.800% Notes [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 1,200,000,000
Proceeds from issuance of debt instrument	CAD 1,569	1,186,000,000
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Interest rate, effective percentage				3.61%
Loss on extinguishment of debt	CAD 47	$ 36,000,000
Interest Rate Swap [Member] | 2.900% 10-year Notes [Member]
Debt Instrument [Line Items]
Repayments of debt			CAD 63	$ 50,000,000