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Leases Lessee, Schedule of Finance Leases (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) tankContainer	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) tankContainer
Lessee, Lease, Description [Line Items]
Number of railcars received | tankContainer	728		21
gatxOperatingLeaseOtherIncome | $	$ 115.8	$ 94.6	$ 81.5

X

- Definition

gatxOperatingLeaseOtherIncome

+ References

No definition available.

+ Details

Name:	gmt_GatxOperatingLeaseOtherIncome
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Number of railcars received

+ References

No definition available.

+ Details

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

X

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 7A
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-7A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-3

+ Details

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