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Legal Proceedings and Other Contingencies Legal Proceedings and Other Contingencies (Details) - Almasy, et al. v. Norfolk Southern Corp $ in Millions | 3 Months Ended
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Mar. 31, 2024 USD ($) individual_resident
Other Commitments [Line Items] |
Number of individual residents | individual_resident | 40
Litigation Settlement, Amount Awarded to Other Party | $ | $ 600

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- Definition Number of individual residents + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Amount awarded to other party in judgment or settlement of litigation. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- 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
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Period Type: | duration

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- Details
Name: | srt_LitigationCaseAxis=gmt_AlmasyEtAl.V.NorfolkSouthernCorpMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |