Description of the Plan (Details) - EBP 004	12 Months Ended
Dec. 31, 2025 USD ($) h	Dec. 31, 2024 USD ($)
EBP, Description of Plan [Line Items]
Minimum service period (in hours) \| h	520
Minimum pretax compensation contribution	1.00%
Maximum pretax compensation contribution	50.00%
Participant compensation matched, percentage	50.00%	50.00%
Percentage of eligible compensation matched	6.00%	6.00%
Vesting percentage	100.00%
Involuntary cash-out limit	$ 1,000
Maximum borrowing, amount	$ 50,000
Maximum borrowing, to total account, percentage	50.00%
Transfers to GATX Corporation Salaried Employees Retirement Savings Plan	$ 129,477	$ 168,289
Minimum
EBP, Description of Plan [Line Items]
Catch-up contributions, percentage	1.00%
Maximum
EBP, Description of Plan [Line Items]
Catch-up contributions, percentage	25.00%

EBP, Contribution, Vesting Percentage

No definition available.

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- Definition

EBP, Involuntary Cash-out Limit

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

EBP, Participant Contribution, Pretax, Annual Compensation to Total Compensation, Catch Up Contributions, Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	gmt_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Participant Contribution, Pretax, Minimum Annual Compensation to Total Compensation, Percentage

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

EBP, Requisite Service Period, Number Of Hours

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of net asset available for benefit transferred to (from) employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) from (to) external plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 962
-SubTopic 325
-Name Accounting Standards Codification
-Section 55
-Paragraph 17
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478873/962-325-55-17

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6A-05(a)(Schedule III))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477804/962-205-S99-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 45
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477063/962-205-45-8

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 962
-SubTopic 205
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478416/962-205-50-1

+ Details

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

- Definition

Percentage of employer matching contribution on eligible participant contribution to employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of participant compensation eligible for employer matching contribution for participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)).

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of maximum account balance that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) can borrow to total participant account balance.

+ References

+ Details

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Namespace Prefix:	us-gaap-ebp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of maximum pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), is permitted to contribute to total pretax annual compensation.

+ References

+ Details

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Period Type:	duration

- Details

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- Details

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