Financial instruments - Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Financial Instruments [Line Items]
Other long-term investments	$ 34,362	$ 37,271
Cross currency swap \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	93,198
Foreign exchange forward \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	0
Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	1,171
Interest rate swaps
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	8,473
Level 1
Fair Value of Financial Instruments [Line Items]
Other long-term investments	814,530
Total financial assets	814,530
Long-term debt	768,400	651,969
Convertible debentures	639	1,018
Total financial liabilities	769,039	652,987
Level 2
Fair Value of Financial Instruments [Line Items]
Notes receivable	110,019	38,192
Derivative financial instruments, assets	970	183
Total financial assets	110,989	38,375
Long-term debt	2,588,373	2,610,742
Convertible debentures	0
Derivative financial instruments, liabilities	102,785	68,867
Total financial liabilities	2,704,861	2,694,733
Level 2 \| Energy contracts \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		109
Level 2 \| Energy contracts \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities		31
Level 2 \| Cross currency swap \| Designated as a hedge \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	93,198	57,412
Level 2 \| Foreign exchange forward \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	869
Level 2 \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	101	74
Derivative financial instruments, liabilities	1,114	2,620
Level 2 \| Interest rate swaps \| Designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	8,473	8,460
Level 2 \| Interest rate swaps \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities		344
Level 2 \| Series C Preferred Stock
Fair Value of Financial Instruments [Line Items]
Convertible debentures		15,124
Preferred shares, Series C	13,703
Level 3
Fair Value of Financial Instruments [Line Items]
Notes receivable	0	0
Derivative financial instruments, assets	61,838	63,363
Total financial assets	61,838	63,363
Derivative financial instruments, liabilities	57	77
Total financial liabilities	57	77
Level 3 \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	61,838	63,363
Derivative financial instruments, liabilities	57	77
Level 3 \| Energy contracts \| Not designated as a hedge \| Cash flow 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
Portion at Other than Fair Value Measurement [Member] \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		441
Carrying amount
Fair Value of Financial Instruments [Line Items]
Notes receivable	103,696	33,378
Other long-term investments	814,530
Derivative financial instruments, assets	62,808	63,546
Total financial assets	981,034	96,924
Long-term debt	3,336,795	3,079,551
Convertible debentures	470	971
Derivative financial instruments, liabilities	102,842	68,944
Total financial liabilities	3,453,525	3,164,184
Carrying amount \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	61,838	63,363
Derivative financial instruments, liabilities	57	77
Carrying amount \| Energy contracts \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		109
Derivative financial instruments, liabilities		31
Carrying amount \| Cross currency swap \| Designated as a hedge \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	93,198	57,412
Carrying amount \| Foreign exchange forward \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	869
Carrying amount \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	101	74
Derivative financial instruments, liabilities	1,114	2,620
Carrying amount \| Interest rate swaps \| Designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	8,473	8,460
Carrying amount \| Interest rate swaps \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities		344
Carrying amount \| Series C Preferred Stock
Fair Value of Financial Instruments [Line Items]
Convertible debentures		14,718
Preferred shares, Series C	13,418
Fair value
Fair Value of Financial Instruments [Line Items]
Notes receivable	110,019	38,192
Other long-term investments	814,530
Derivative financial instruments, assets	62,808	63,546
Total financial assets	987,357	101,738
Long-term debt	3,356,773	3,262,711
Convertible debentures	639	1,018
Derivative financial instruments, liabilities	102,842	68,944
Total financial liabilities	3,473,957	3,347,797
Fair value \| Energy contracts \| Designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	61,838	63,363
Derivative financial instruments, liabilities	57	77
Fair value \| Energy contracts \| Not designated as a hedge \| Cash flow hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets		109
Derivative financial instruments, liabilities		31
Fair value \| Cross currency swap \| Designated as a hedge \| Net Investment Hedging
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	93,198	57,412
Fair value \| Foreign exchange forward \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	869
Fair value \| Commodity contracts for regulatory operations
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, assets	101	74
Derivative financial instruments, liabilities	1,114	2,620
Fair value \| Interest rate swaps \| Designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities	8,473	8,460
Fair value \| Interest rate swaps \| Not designated as a hedge
Fair Value of Financial Instruments [Line Items]
Derivative financial instruments, liabilities		344
Fair value \| Series C Preferred Stock
Fair Value of Financial Instruments [Line Items]
Convertible debentures		$ 15,124
Preferred shares, Series C	$ 13,703

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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-Publisher FASB
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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.

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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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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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- Definition

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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