XML 121 R41.htm IDEA: XBRL DOCUMENT
v3.20.2

Fair Value Disclosure Fair Value Disclosure - Other Financial Instruments (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Reported Value Measurement [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Recourse Fixed Rate Debt Fair Value Disclosure | $ 4,788.3 | $ 4,389.3
Recourse Floating Rate Debt Fair Value Disclosure | 422.9 | 417.5
Estimate of Fair Value Measurement [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Recourse Fixed Rate Debt Fair Value Disclosure | 5,295.4 | 4,644.6
Recourse Floating Rate Debt Fair Value Disclosure | $ 419.7 | $ 419.0

X
- Definition Recourse fixed rate debt fair value disclosure. + References No definition available. + Details
Name: | gmt_RecourseFixedRateDebtFairValueDisclosure
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Recourse floating rate debt fair value disclosure. + References No definition available. + Details
Name: | gmt_RecourseFloatingRateDebtFairValueDisclosure
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Details
Name: | us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |