XML 42 R28.htm IDEA: XBRL DOCUMENT
v3.23.2

Description of Business (Details) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2023 USD ($) AircraftEngines | Mar. 31, 2023 USD ($) AircraftEngines | Sep. 30, 2022 AircraftEngines | Jun. 30, 2022 USD ($) AircraftEngines | Jun. 30, 2023 USD ($) AircraftEngines Segment | Jun. 30, 2022 USD ($) AircraftEngines | Dec. 31, 2022 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Number of business segments | Segment |  |  |  |  | 3 |  |
Net gain on asset dispositions | $ 41.1 |  |  | $ (24.2) | $ 88.2 | $ 49.5 |
Number of Specialized Gas Vessels Sold | AircraftEngines | 1 | 1 |  |  |  |  |
Asset Impairment Charges | $ 0.0 |  |  | 31.5 | 1.2 | 31.5 |
Portfolio Investments and Capital Additions | $ 486.6 |  |  | $ 314.1 | $ 873.6 | $ 684.5 |
Number of Aircraft Engines Acquired | AircraftEngines | 9 |  |  |  |  |  |
Number of Specialized Gas Vessels | AircraftEngines | 1 | 2 |  | 5 | 1 | 5 |
Portfolio Management |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Number of Specialized Gas Vessels Sold | AircraftEngines |  | 1 | 2 |  |  |  |
Asset Impairment Charges | $ 0.0 | $ 1.2 |  | $ 31.5 | $ 1.2 | $ 31.5 | $ 34.3
Portfolio Investments and Capital Additions | $ 239.0 |  |  | 0.0 | 239.0 | 0.0 |
Number of Aircraft Engines Acquired | AircraftEngines | 9 |  |  |  |  |  |
Rail International |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  | 0.0 |  | $ 14.6
Portfolio Investments and Capital Additions | $ 77.3 |  |  | $ 48.8 | $ 158.4 | $ 127.7 |

X
- Definition Number of Aircraft Engines Acquired + References No definition available. + Details
Name: | gmt_NumberOfAircraftEnginesAcquired
Namespace Prefix: | gmt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Specialized Gas Vessels Sold + References No definition available. + Details
Name: | gmt_NumberOfSpecializedGasVesselsSold
Namespace Prefix: | gmt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Specialized Gas Vessels + References No definition available. + Details
Name: | gmt_NumberofSpecializedGasVessels
Namespace Prefix: | gmt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Portfolio Investments and Capital Additions + References No definition available. + Details
Name: | gmt_PortfolioInvestmentsandCapitalAdditions
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -SubTopic 10 -Topic 230 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482130/360-10-45-4 + Details
Name: | us-gaap_AssetImpairmentCharges
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of gain (loss) from the difference between the sale price or salvage price and the book value of an asset that was sold or retired, and gain (loss) from the write down of assets from their carrying value to fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 28 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482740/230-10-45-28 + Details
Name: | us-gaap_GainLossOnSalesOfAssetsAndAssetImpairmentCharges
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 280 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482810/280-10-50-18 + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(g)(1)(ii)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 323 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481687/323-10-50-3 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 28 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482907/825-10-50-28 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 323 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481687/323-10-50-3 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 323 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481687/323-10-50-3 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 323 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481687/323-10-50-3 + Details
Name: | us-gaap_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=gmt_PortfolioManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=gmt_RailInternationalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |