LEASES (Schedule of lease cost) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Lessee, Lease, Description [Line Items]
Operating lease, cost	$ 21,214	$ 7,134	$ 44,096	$ 25,906
Total lease cost	1,172	4,011	7,444	16,266
Vessel/Fleet [Member] \| Location, Statement of Income, Balance [Axis]: us-gaap:OperatingLeaseCost
Lessee, Lease, Description [Line Items]
Operating lease, cost	12	2,494	3,051	11,714
Short-term lease, cost	867	1,244	3,514	3,795
Office space and vessel equipment \| Location, Statement of Income, Balance [Axis]: us-gaap:GeneralAndAdministrativeExpense
Lessee, Lease, Description [Line Items]
Operating lease, cost	227	227	681	681
Office space and vessel equipment \| Location, Statement of Income, Balance [Axis]: insw:VesselExpenses
Lessee, Lease, Description [Line Items]
Operating lease, cost	20	0	60	0
Office space and vessel equipment \| Location, Statement of Income, Balance [Axis]: insw:VoyageExpenses
Lessee, Lease, Description [Line Items]
Operating lease, cost	46	46	138	76
Lightering Services Component [Member] \| Vessel/Fleet [Member]
Lessee, Lease, Description [Line Items]
Short-term lease, cost	$ 200	$ 400	$ 1,600	$ 1,100

Amount of lease cost recognized by lessee for lease contract.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of single lease cost, calculated by allocation of remaining cost of lease over remaining lease term. Includes, but is not limited to, single lease cost, after impairment of right-of-use asset, calculated by amortization of remaining right-of-use asset and accretion of lease liability.

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Amount of short-term lease cost, excluding expense for lease with term of one month or less.

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