FINANCIAL INSTRUMENTS (Schedule of carrying value and estimated fair value of financial assets and liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financial Assets And Liabilities [Line Items]
Available-for-sale debt securities	$ 8,293	$ 9,431
Equity securities	14,512	19,374
Equity securities pledged to creditors	13,032	9,007
Derivatives:
Derivative instrument, long-term assets	3,675	3,406
Carrying value \| Fair Value, Measurements, Recurring
Financial Assets And Liabilities [Line Items]
Available-for-sale debt securities	8,293	9,431
Equity securities	1,480	10,367
Equity securities pledged to creditors	13,032	9,007
Derivatives:
Derivative instrument, long-term assets	3,675	3,406
Interest rate/currency swap contracts - short-term payables	(1,714)	(1,572)
Interest rate/currency swaps, long-term payables	(21,357)	(32,712)
Carrying value \| Fair Value, Measurements, Recurring \| Floating rate NOK bonds due 2023
Financial Assets And Liabilities [Line Items]
Long-term debt	(81,238)	(81,572)
Carrying value \| Fair Value, Measurements, Recurring \| Floating rate NOK bonds due 2024
Financial Assets And Liabilities [Line Items]
Long-term debt	(80,657)	(80,989)
Carrying value \| Fair Value, Measurements, Recurring \| Floating rate NOK bonds due 2025
Financial Assets And Liabilities [Line Items]
Long-term debt	(62,669)	(62,927)
Carrying value \| Fair Value, Measurements, Recurring \| 5.75% senior unsecured convertible notes due 2021
Financial Assets And Liabilities [Line Items]
Long-term debt	(146,919)	(212,230)
Carrying value \| Fair Value, Measurements, Recurring \| 4.875% senior unsecured convertible notes due 2023
Financial Assets And Liabilities [Line Items]
Long-term debt	(137,900)	(139,900)
Carrying value \| Fair Value, Measurements, Recurring \| 7.25% senior unsecured sustainability-linked bonds due 2026
Financial Assets And Liabilities [Line Items]
Long-term debt	(150,000)	0
Fair value \| Fair Value, Measurements, Recurring
Financial Assets And Liabilities [Line Items]
Available-for-sale debt securities	8,293	9,431
Equity securities	1,480	10,367
Equity securities pledged to creditors	13,032	9,007
Derivatives:
Derivative instrument, long-term assets	3,675	3,406
Interest rate/currency swap contracts - short-term payables	(1,714)	(1,572)
Interest rate/currency swaps, long-term payables	(21,357)	(32,712)
Fair value \| Fair Value, Measurements, Recurring \| Floating rate NOK bonds due 2023
Financial Assets And Liabilities [Line Items]
Long-term debt	(80,831)	(78,513)
Fair value \| Fair Value, Measurements, Recurring \| Floating rate NOK bonds due 2024
Financial Assets And Liabilities [Line Items]
Long-term debt	(80,052)	(76,940)
Fair value \| Fair Value, Measurements, Recurring \| Floating rate NOK bonds due 2025
Financial Assets And Liabilities [Line Items]
Long-term debt	(60,632)	(57,421)
Fair value \| Fair Value, Measurements, Recurring \| 5.75% senior unsecured convertible notes due 2021
Financial Assets And Liabilities [Line Items]
Long-term debt	(145,450)	(199,496)
Fair value \| Fair Value, Measurements, Recurring \| 4.875% senior unsecured convertible notes due 2023
Financial Assets And Liabilities [Line Items]
Long-term debt	(137,470)	(123,112)
Fair value \| Fair Value, Measurements, Recurring \| 7.25% senior unsecured sustainability-linked bonds due 2026
Financial Assets And Liabilities [Line Items]
Long-term debt	$ (151,125)	$ 0

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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