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v3.20.2

Basis of Presentation Pension and Post-Retirement Benefits (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |
Right of use assets | $ 364.3 | $ 420.6
Lease liability | $ 368.0 | $ 437.3

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- Definition Lease, Liability + References No definition available. + Details
Name: | gmt_LeaseLiability
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Lease, Right-of-Use Asset + References No definition available. + Details
Name: | gmt_LeaseRightofUseAsset
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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