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Investments in Affiliated Companies (Summarized Financial Data-RRPF) (Details 6) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Statement [Abstract] |  |  |  |  |  |
Lease revenue | $ 339.0 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue |  | $ 334.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue |  | $ 651.8 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue |
Income taxes |  |  | (15.2)gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax |  | (9.2)gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax |
Net income | 99.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss |  | 108.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss |  | 75.3 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss |
Assets [Abstract] |  |  |  |  |  |
Current assets | 211.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentAssets |  | 200.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentAssets |  |  |
Other noncurrent assets | 3,195.1 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentAssets |  | 2,989.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentAssets |  |  |
Total assets | 3,406.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets |  | 3,189.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets |  |  |
Liabilities and Equity [Abstract] |  |  |  |  |  |
Current liabilities | 378.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentLiabilities |  | 159.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentLiabilities |  |  |
Long-term debt | 2,388.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities |  | 2,408.8 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities |  |  |
Shareholders’ equity | 640.1 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital |  | 621.3 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital |  |  |
Total liabilities and shareholders' equity | 3,406.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilitiesAndEquity |  | 3,189.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilitiesAndEquity |  |  |
RRPF Joint Ventures [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
number of aircraft engines | 433 gmt_NumberofAircraftEngines / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |  |  |
Income Statement [Abstract] |  |  |  |  |  |
Depreciation expense | (140.7)gmt_EquityMethodInvestmentSummarizedFinancialInformationDepreciation / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | (120.2)gmt_EquityMethodInvestmentSummarizedFinancialInformationDepreciation / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | (115.1)gmt_EquityMethodInvestmentSummarizedFinancialInformationDepreciation / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
Interest expense | (59.5)gmt_EquityMethodInvestmentSummarizedFinancialInformationInterestExpense / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | (65.8)gmt_EquityMethodInvestmentSummarizedFinancialInformationInterestExpense / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | (64.8)gmt_EquityMethodInvestmentSummarizedFinancialInformationInterestExpense / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
Other expenses | (11.8)gmt_EquityMethodInvestmentSummarizedFinancialInformationOtherExpenses / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | (10.2)gmt_EquityMethodInvestmentSummarizedFinancialInformationOtherExpenses / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | (16.3)gmt_EquityMethodInvestmentSummarizedFinancialInformationOtherExpenses / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
Gains on sales of assets | 22.7 gmt_EquityMethodInvestmentSummarizedFinancialInformationGainsLosses / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 35.2 gmt_EquityMethodInvestmentSummarizedFinancialInformationGainsLosses / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 35.9 gmt_EquityMethodInvestmentSummarizedFinancialInformationGainsLosses / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
Income before income taxes | 112.3 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeBeforeIncomeTax / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 106.3 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeBeforeIncomeTax / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 93.0 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeBeforeIncomeTax / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
Income taxes | (17.3)gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember | [1] | 0.4 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember | [1] | (3.8)gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember | [1]
Net income | 95.0 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 106.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 89.2 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
Assets [Abstract] |  |  |  |  |  |
Current assets | 146.3 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentAssets / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 151.0 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentAssets / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
Operating assets, net of accumulated depreciation of $744.5 and $651.8 | 2,988.1 gmt_EquityMethodInvestmentsSummarizedFinancialInformationOperatingAssetsNet / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember | [2] | 2,619.6 gmt_EquityMethodInvestmentsSummarizedFinancialInformationOperatingAssetsNet / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember | [2] |  |
Total assets | 3,134.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 2,770.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
Liabilities and Equity [Abstract] |  |  |  |  |  |
Current liabilities | 358.5 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentLiabilities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 141.0 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCurrentLiabilities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
Long-term debt | 2,012.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 1,963.1 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNoncurrentLiabilities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
Other liabilities | 218.1 gmt_EquityMethodInvestmentSummarizedFinancialInformationOtherLiabilities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 212.4 gmt_EquityMethodInvestmentSummarizedFinancialInformationOtherLiabilities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
Shareholders’ equity | 544.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 454.1 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquityOrCapital / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
Total liabilities and shareholders' equity | 3,134.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilitiesAndEquity / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 2,770.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilitiesAndEquity / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  |  |
RRPF Joint Ventures [Member] | Third Parties [Member] |  |  |  |  |  |
Income Statement [Abstract] |  |  |  |  |  |
Lease revenue | 176.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / dei_LegalEntityAxis = gmt_ThirdPartiesMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 147.6 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / dei_LegalEntityAxis = gmt_ThirdPartiesMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | 140.4 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / dei_LegalEntityAxis = gmt_ThirdPartiesMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
RRPF Joint Ventures [Member] | Rolls-Royce [Member] |  |  |  |  |  |
Income Statement [Abstract] |  |  |  |  |  |
Lease revenue | $ 124.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / dei_LegalEntityAxis = gmt_RollsRoyceMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | $ 119.7 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / dei_LegalEntityAxis = gmt_RollsRoyceMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |  | $ 112.9 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue / dei_LegalEntityAxis = gmt_RollsRoyceMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gmt_RrpfJointVenturesMember |
[1] | (1) Represents income taxes directly attributable to the RRPF affiliates in the United Kingdom. Several of the RRPF affiliates are flow through entities and income taxes are incurred at the shareholder level. Amounts shown for 2013 and 2012 include tax benefits of approximately $15.2 million and $9.2 million, attributable to statutory rate decreases in the United Kingdom.
[2] | (a) All operating assets were pledged as collateral for long-term debt obligations.

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