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Allowance for Losses (Details Textual) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Allowance for Losses (Textual) [Abstract] |  |  |
Allowances for trade receivables | $ (5.7)us-gaap_LoansAndLeasesReceivableAllowance | $ (5.2)us-gaap_LoansAndLeasesReceivableAllowance | $ (4.6)us-gaap_LoansAndLeasesReceivableAllowance
Allowance for Trade Receivables [Member] |  |  |
Allowance for Losses (Textual) [Abstract] |  |  |
Allowances for trade receivables | (4.5)us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_AllowanceForTradeReceivablesMember | (4.0)us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_AllowanceForTradeReceivablesMember |
Allowances for trade receivables as percentage of rent and other receivables | 5.20%gmt_AllowancesForTradeReceivablesAsPercentageOfRentAndOtherReceivables / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_AllowanceForTradeReceivablesMember | 4.90%gmt_AllowancesForTradeReceivablesAsPercentageOfRentAndOtherReceivables / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_AllowanceForTradeReceivablesMember |
Allowance for Finance Leases [Member] |  |  |
Allowance for Losses (Textual) [Abstract] |  |  |
Allowances for trade receivables | $ (1.2)us-gaap_LoansAndLeasesReceivableAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_AllowanceForLoanAndLeaseLossesMember |  |

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- Definition Allowances for trade receivables as percentage of rent and other receivables. + References No definition available. + Details
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- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph d -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section L -Subsection 1 + Details
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