Leases - Maturity Schedule (Details) $ in Millions	Dec. 31, 2021 USD ($)
Lessee, Operating Lease, Description [Abstract]
2022	$ 968.3
2023	753.3
2024	546.4
2025	330.8
2026	195.9
Years thereafter	328.6
Total future receipts from leases	3,123.3
Finance Lease Liability [Abstract]
Lessor, Finance Lease, Payment to be Received, Year One	36.6
Lessor, Finance Lease, Payment to be Received, Year Two	15.5
Lessor, Finance Lease, Payment to be Received, Year Three	16.1
Lessor, Finance Lease, Payment to be Received, Year Four	12.6
Lessor, Finance Lease, Payment to be Received, Year Five	11.6
Lessor, Finance Lease, Payment to be Received, After Year Five	14.5
Lessor, Finance Lease, Payments to be Received	106.9
2022	1,004.9
2023	768.8
2024	562.5
2025	343.4
2026	207.5
Years thereafter	343.1
Leases Future Minimum Payments Receivable	$ 3,230.2

Leases future minimum payments receivable.

No definition available.

Name:	gmt_LeasesFutureMinimumPaymentsReceivable
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leases future minimum payments receivable current.

+ References

No definition available.

+ Details

Name:	gmt_LeasesFutureMinimumPaymentsReceivableCurrent
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leases future minimum payments receivable in five years.

+ References

No definition available.

+ Details

Name:	gmt_LeasesFutureMinimumPaymentsReceivableInFiveYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leases future minimum payments receivable in four years.

+ References

No definition available.

+ Details

Name:	gmt_LeasesFutureMinimumPaymentsReceivableInFourYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leases future minimum payments receivable in three years.

+ References

No definition available.

+ Details

Name:	gmt_LeasesFutureMinimumPaymentsReceivableInThreeYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leases future minimum payments receivable in two years.

+ References

No definition available.

+ Details

Name:	gmt_LeasesFutureMinimumPaymentsReceivableInTwoYears
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Leases future minimum payments receivable thereafter.

+ References

No definition available.

+ Details

Name:	gmt_LeasesFutureMinimumPaymentsReceivableThereafter
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payment to be Received, After Year Five

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentToBeReceivedAfterYearFive
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payment to be Received, Year Five

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentToBeReceivedYearFive
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payment to be Received, Year Four

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentToBeReceivedYearFour
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payment to be Received, Year One

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentToBeReceivedYearOne
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payment to be Received, Year Three

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentToBeReceivedYearThree
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payment to be Received, Year Two

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentToBeReceivedYearTwo
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessor, Finance Lease, Payments to be Received

+ References

No definition available.

+ Details

Name:	gmt_LessorFinanceLeasePaymentsToBeReceived
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_LesseeOperatingLeaseDescriptionAbstract
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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=124258985&loc=SL77919396-209981

+ 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