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Leases (Details Textual) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 Corporations | Dec. 31, 2013 | Dec. 31, 2012
Operating Leased Assets [Line Items] |  |  |
Schedule of Capital Leased Assets [Table Text Block] | The following table shows assets we financed with capital lease obligations as of December 31 (in millions):
| 2014 |  | 2013
Marine vessels | $ | 83.1 |  |  | $ | 81.4 |
Less: allowance for depreciation | (75.3 | ) |  | (71.4 | )
| $ | 7.8 |  |  | $ | 10.0 |
Operating Leases, Rent Expense | $ 112.9 us-gaap_LeaseAndRentalExpense | $ 134.0 us-gaap_LeaseAndRentalExpense | $ 133.3 us-gaap_LeaseAndRentalExpense
Leases (Textual) [Abstract] |  |  |
Income from leveraged leases, net of taxes |  |  | 0.2 us-gaap_LeveragedLeasesIncomeStatementNetIncomeFromLeveragedLeases
Rental income from usage rents | 83.9 us-gaap_OperatingLeasesIncomeStatementContingentRevenue | 21.3 us-gaap_OperatingLeasesIncomeStatementContingentRevenue | 18.2 us-gaap_OperatingLeasesIncomeStatementContingentRevenue
Property, Plant and Equipment, Schedule of Significant Acquisitions and Disposals [Table Text Block] | 18500 |  |
Deferred initial direct costs of direct financial leases | $ 0.3 us-gaap_DeferredCostsLeasingGross | $ 0.4 us-gaap_DeferredCostsLeasingGross |
Number of corporations | 1 gmt_NumberOfCorporations |  |
Railroad Transportation Equipment [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Schedule of Capital Leased Assets [Table Text Block] | 23000 |  |
Marine vessels [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Schedule of Capital Leased Assets [Table Text Block] | 1 |  |

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- Definition For an unclassified balance sheet, this element represents costs incurred by the lessor that are (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. This amount is before considering accumulated amortization representing the periodic charge to earnings to recognize the deferred costs over the term of the related lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=6748888&loc=d3e40246-112709 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=6748888&loc=d3e40588-112709 + Details
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- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
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- Definition The total recognized during the period for income on leases meeting the criteria for classification as leveraged leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=7662751&loc=d3e45152-112736 + Details
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- Definition Amount of contingent rental revenue recognized for the period under lease, based on the occurrences of an event or condition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition Tabular disclosure of all information related to any significant acquisition and disposal. Disclosure may include methodology and assumptions, type of asset, asset classification, useful life, useful purpose, acquisition cost, method of acquisition or disposal, depreciation method, gain (loss) on disposal pretax and net of tax, date of acquisition or disposal and restrictions on amount of proceeds from donated assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
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- Definition Tabular disclosure of long-lived, depreciable assets that are subject to a lease meeting the criteria for capitalization and are used in the normal conduct of business to produce goods and services. Examples may include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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