Leases Future Contractual Receipts (Details) $ in Millions	Dec. 31, 2025 USD ($)
Lessee, Lease, Description [Line Items]
2026	$ 1,315.1
2027	1,071.1
2028	854.3
2029	641.3
2030	456.0
Thereafter	836.8
Total future receipts from leases	5,174.6
RRPF Joint Ventures [Member]
Lessee, Lease, Description [Line Items]
2026	588.5
2027	475.3
2028	395.0
2029	338.4
2030	286.2
Thereafter	934.7
Total future receipts from leases	3,018.1
RRPF Joint Ventures [Member] \| Rolls-Royce [Member]
Lessee, Lease, Description [Line Items]
2026	180.4
2027	102.8
2028	68.9
2029	56.5
2030	47.5
Thereafter	73.4
Total future receipts from leases	529.5
RRPF Joint Ventures [Member] \| Third Parties [Member]
Lessee, Lease, Description [Line Items]
2026	408.1
2027	372.5
2028	326.1
2029	281.9
2030	238.7
Thereafter	861.3
Total future receipts from leases	$ 2,488.6

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.

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

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

- Definition

Amount of lease payments to be received by lessor for operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479773/842-30-50-12

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceived
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of lease payment to be received by lessor for operating lease in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceivedFiveYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of lease payment to be received by lessor for operating lease in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceivedFourYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of lease payment to be received by lessor for operating lease in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceivedNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of lease payment to be received by lessor for operating lease after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceivedThereafter
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of lease payment to be received by lessor for operating lease in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceivedThreeYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of lease payment to be received by lessor for operating lease in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LessorOperatingLeasePaymentsToBeReceivedTwoYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=gmt_RrpfJointVenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gmt_RollsRoyceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gmt_ThirdPartiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: