Supplemental Cash Flow and Noncash Investing Transactions (Details Textual) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) Car	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Car
Noncash or Part Noncash Divestitures [Line Items]
Portfolio proceeds	$ 165.6	$ 223.7	$ 482.2
Capital Lease Obligations Incurred	$ 0.0		14.8
Supplemental Cash Flow and Noncash Investing and Financing Transactions (Textual) [Abstract]
Aggregate fair value of liquidating distributions			$ 17.8
Number of railcars received \| Car	1,224		157
Payments for (Proceeds from) Productive Assets	$ 20.7
Noncash or Part Noncash Acquisition, Debt Assumed	$ 11.6

Number of railcars received

No definition available.

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

- Definition

Portfolio proceeds.

+ References

No definition available.

+ Details

Name:	gmt_PortfolioProceeds
Namespace Prefix:	gmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Supplemental Cash Flow and Noncash Investing and Financing Transactions.

+ References

No definition available.

+ Details

Name:	gmt_SupplementalCashFlowAndNoncashInvestingAndFinancingTransactionsTextualAbstract
Namespace Prefix:	gmt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The increase during the period in capital lease obligations due to entering into new capital leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

+ Details

Name:	us-gaap_CapitalLeaseObligationsIncurred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The fair value of assets acquired in noncash investing or financing activities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

+ Details

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

- Definition

The amount of debt that an Entity assumes in acquiring a business or in consideration for an asset received in a noncash (or part noncash) acquisition. Noncash is defined as transactions during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

+ Details

Name:	us-gaap_NoncashOrPartNoncashAcquisitionDebtAssumed1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

- Definition

The net cash outflow or inflow from purchases, sales and disposals of property, plant and equipment and other productive assets, including intangibles.

+ References

No definition available.

+ Details

Name:	us-gaap_PaymentsForProceedsFromProductiveAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration