Leases (Details Textual) $ in Millions	12 Months Ended
Dec. 31, 2024 USD ($) tankContainer railcar	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) tankContainer
Operating Leased Assets [Line Items]
Lessee, Lease, Number of Railcars Leased \| railcar	4,432,000,000
Sublease Income	$ 41.9	$ 41.3	$ 40.1
Number of railcars received \| tankContainer	728		21
Payments to Acquire Equipment on Lease	$ 30.4		$ 1.5

Lessee, Lease, Number of Railcars Leased

No definition available.

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Namespace Prefix:	gmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of railcars received

+ References

No definition available.

+ Details

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Namespace Prefix:	gmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow for payments to acquire rented equipment which is recorded as an asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-13

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Amount of sublease income excluding finance and operating lease expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 53
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479589/842-20-55-53

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-4

+ Details

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Balance Type:	credit
Period Type:	duration