Financial instruments - Fair Value of Financial Instruments (Detail) - CAD CAD in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cross currency swap \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	CAD 72,023
Foreign exchange forward \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	432
Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	3,422
Interest rate swaps
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	10,613
Level 1
Fair Value of Financial Instruments [Line Items]
Long-term debt	817,895	CAD 517,637
Convertible debentures	1,277	455,975
Total financial liabilities	819,172	973,612
Level 2
Fair Value of Financial Instruments [Line Items]
Notes receivable	47,912	47,933
Derivative financial instruments, assets	8,041	66,316
Total financial assets	55,953	114,249
Long-term debt	3,275,176	3,481,629
Convertible debentures	0
Derivative financial instruments, liabilities	86,393	108,825
Total financial liabilities	3,380,542	3,609,067
Level 2 \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	0
Level 2 \| Energy contracts \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	137
Level 2 \| Energy contracts \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	39
Level 2 \| Cross currency swap \| Designated as a hedge \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	72,023	95,404
Level 2 \| Foreign exchange forward \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	432
Level 2 \| Foreign exchange forward \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		17,864
Level 2 \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	92	359
Derivative financial instruments, liabilities	3,286	36
Level 2 \| Transmission congestion rights
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	7,812
Level 2 \| Interest rate swaps \| Designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	10,613	13,385
Level 2 \| Interest rate swaps \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		48,093
Level 2 \| Series C Preferred Stock
Fair Value of Financial Instruments [Line Items]
Convertible debentures		18,613
Preferred shares, Series C	18,973
Level 3
Fair Value of Financial Instruments [Line Items]
Notes receivable	0	0
Derivative financial instruments, assets	79,490	84,554
Total financial assets	79,490	84,554
Derivative financial instruments, liabilities	97	0
Total financial liabilities	97	0
Level 3 \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	79,490	84,554
Derivative financial instruments, liabilities	97
Level 3 \| Energy contracts \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	0
Level 3 \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	0
Level 3 \| Transmission congestion rights
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	0
Portion at Other than Fair Value Measurement [Member] \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	553	314
Carrying amount
Fair Value of Financial Instruments [Line Items]
Notes receivable	41,873	38,183
Derivative financial instruments, assets	87,531	150,870
Total financial assets	129,404	189,053
Long-term debt	3,863,296	3,913,415
Convertible debentures	1,218	358,619
Derivative financial instruments, liabilities	86,490	108,825
Total financial liabilities	3,969,468	4,399,319
Carrying amount \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	79,490	84,554
Derivative financial instruments, liabilities	97
Carrying amount \| Energy contracts \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	137
Carrying amount \| Energy contracts \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	39
Carrying amount \| Cross currency swap \| Designated as a hedge \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	72,023	95,404
Carrying amount \| Foreign exchange forward \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	432
Carrying amount \| Foreign exchange forward \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		17,864
Carrying amount \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	92	359
Derivative financial instruments, liabilities	3,286	36
Carrying amount \| Transmission congestion rights
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	7,812
Carrying amount \| Interest rate swaps \| Designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	10,613	13,385
Carrying amount \| Interest rate swaps \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		48,093
Carrying amount \| Series C Preferred Stock
Fair Value of Financial Instruments [Line Items]
Convertible debentures		18,460
Preferred shares, Series C	18,464
Fair Value
Fair Value of Financial Instruments [Line Items]
Notes receivable	47,912	47,933
Derivative financial instruments, assets	87,531	150,870
Total financial assets	135,443	198,803
Long-term debt	4,093,071	3,999,266
Convertible debentures	1,277	455,975
Derivative financial instruments, liabilities	86,490	108,825
Total financial liabilities	4,199,811	4,582,679
Fair Value \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	79,490	84,554
Derivative financial instruments, liabilities	97
Fair Value \| Energy contracts \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	137
Fair Value \| Energy contracts \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	39
Fair Value \| Cross currency swap \| Designated as a hedge \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	72,023	95,404
Fair Value \| Foreign exchange forward \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	432
Fair Value \| Foreign exchange forward \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		17,864
Fair Value \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	92	359
Derivative financial instruments, liabilities	3,286	36
Fair Value \| Transmission congestion rights
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	7,812
Fair Value \| Interest rate swaps \| Designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	10,613	13,385
Fair Value \| Interest rate swaps \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		48,093
Fair Value \| Series C Preferred Stock
Fair Value of Financial Instruments [Line Items]
Convertible debentures		CAD 18,613
Preferred shares, Series C	CAD 18,973

Fair Value of Financial Instruments [Line Items]

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

+ References

No definition available.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

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Fair value portion of shares that an entity is required to redeem for cash or other assets at a fixed or determinable date or upon the occurrence of an event.

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Fair value portion of an agreement for an unconditional promise by the maker to pay the holder a definite sum of money at a future date.

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