Administrative and judicial proceedings at sites.

No definition available.

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

- Definition

Number of railcars

+ References

No definition available.

+ Details

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

- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

+ Details

Name:	us-gaap_AccrualForEnvironmentalLossContingencies
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

+ Details

Name:	us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.25)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LitigationReserve
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

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

- Definition

Number of claims dismissed.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingencyClaimsDismissedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of claims settled.

+ References

No definition available.

+ Details

Name:	us-gaap_LossContingencyClaimsSettledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The total number of new claims filed pertaining to a loss contingency during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyNewClaimsFiledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of pending claims pertaining to a loss contingency.

+ References

+ Details

Name:	us-gaap_LossContingencyPendingClaimsNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=gmt_ViareggioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_AsbestosIssueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_PendingLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=gmt_AsbestosCasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gmt_LitigationCaseDetailsAxis=gmt_PuncturedAndIgnitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gmt_LitigationCaseDetailsAxis=gmt_OverturnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gmt_LitigationCaseDetailsAxis=gmt_DerailedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Legal Proceedings and Other Contingencies (Details) € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 EUR (€) case Site	Dec. 31, 2017 USD ($) case	Dec. 31, 2016 USD ($)	Jun. 29, 2009 Car
Loss Contingencies [Line Items]
Accruals for losses related to probable litigation matters \| $		$ 3.7
Number of sites for which the Company is involved in environmental remediation \| Site	16,000,000
Accruals for remediation and restoration \| $		$ 13.1	$ 14.9
Viareggio [Member]
Loss Contingencies [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims \| €	€ 1.4
Asbestos - related cases pending against the company and its subsidiaries		56
Asbestos Issue [Member]
Loss Contingencies [Line Items]
New cases filed number	3
Loss Contingency, Claims Dismissed, Number	5
Loss Contingency, Claims Settled, Number	3
Pending Litigation [Member] \| Asbestos Cases [Member]
Loss Contingencies [Line Items]
Asbestos - related cases pending against the company and its subsidiaries		17
Punctured and Ignited [Member] \| Viareggio [Member]
Loss Contingencies [Line Items]
number of railcars \| Car				1
Overturned [Member] \| Viareggio [Member]
Loss Contingencies [Line Items]
number of railcars \| Car				5
Derailed [Member] \| Viareggio [Member]
Loss Contingencies [Line Items]
number of railcars \| Car				14