Fair Value Of Financial Instruments Derivatives And Fair Value Disclosures [Line Items].

No definition available.

Name:	insw_FairValueOfFinancialInstrumentsDerivativesAndFairValueDisclosuresLineItems
Namespace Prefix:	insw_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Average floor rate on a group of interest rate derivatives, such as interest rate floor or collars. If market rates fall below the floor rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageFloorInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of increase (decrease) in fair value of other financial instruments used in operating activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInOtherFinancialInstrumentsUsedInOperatingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash outflow for cost from early extinguishment and prepayment of debt. Includes, but is not limited to, third-party cost, premium paid, and other fee paid to lender directly for debt extinguishment or debt prepayment. Excludes accrued interest.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsOfDebtExtinguishmentCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=insw_SinosureCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

FAIR VALUE OF FINANCIAL INSTRUMENTS, DERIVATIVES AND FAIR VALUE DISCLOSURES (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 27, 2020	Mar. 31, 2020	Apr. 30, 2020	Jan. 28, 2020
Fair Value Of Financial Instruments Derivatives And Fair Value Disclosures [Line Items]
Extinguishment of debt		$ 382,699
Change in fair value of interest rate collar recorded through earnings		$ (1,271)
Interest Rate Cap/Collar [Member]
Fair Value Of Financial Instruments Derivatives And Fair Value Disclosures [Line Items]
Derivative, notional amount	$ 350,000
Change in fair value of interest rate collar recorded through earnings	$ 1,271
Interest Rate Swap [Member]
Fair Value Of Financial Instruments Derivatives And Fair Value Disclosures [Line Items]
Derivative, notional amount				$ 250,000
Derivative, fixed interest rate				1.97%
Average floor rate				0.00%
Subsequent Event [Member]
Fair Value Of Financial Instruments Derivatives And Fair Value Disclosures [Line Items]
Derivative, notional amount			$ 25,000
Derivative, fixed interest rate			0.50%
Sinosure Credit Facility [Member] \| Interest Rate Swap [Member]
Fair Value Of Financial Instruments Derivatives And Fair Value Disclosures [Line Items]
Derivative, fixed interest rate		2.76%