Debt (Senior and Subordinated Notes) (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Debt instrument covenant limitation on total borrowings percentage of total assets	70.00%
Debt instrument covenant net worth	$ 600,000
8.5% Senior Notes [Member]
Debt instrument, interest rate, stated percentage	8.50%
10.75% Subordinated Notes [Member]
Debt instrument, interest rate, stated percentage	10.75%
Senior Notes [Member] \| 8.5% Senior Notes [Member]
Debt instrument, interest rate, stated percentage	8.50%	8.50%
Subordinated Debt [Member] \| 10.75% Subordinated Notes [Member]
Debt instrument, interest rate, stated percentage	10.75%	10.75%
Debt instrument, covenant compliance	The Company was in compliance with the financial covenants under all of its debt facilities as of September 30, 2019.

Debt instrument covenant limitation on total borrowings percentage of total assets.

No definition available.

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Period Type:	instant

- Definition

Debt instrument covenant net worth.

+ References

No definition available.

+ Details

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Namespace Prefix:	insw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

States whether the entity was in compliance with the debt covenants throughout the reporting period, and describes facts and circumstances of any compliance failure.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Details

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- Details

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- Details

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- Details

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