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

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- Definition

Lessee, Lease, Number of Railcars Leased

+ References

No definition available.

+ Details

Name:	gmt_LesseeLeaseNumberofRailcarsLeased
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