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Leases - Narrative (Details) | Mar. 31, 2019 railcar
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Leases [Abstract] |
Number of railcars leased | 8,400

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- Definition Lessee, Lease, Number of Railcars Leased + References No definition available. + Details
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Namespace Prefix: | gmt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LeasesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration