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Fair Value Disclosure (Details Textual) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013
Fair Value Disclosure (Textual) [Abstract] |  |
Derivative, Notional Amount | $ 281.5 invest_DerivativeNotionalAmount | $ 0 invest_DerivativeNotionalAmount
Cash Flow Hedge Pre Tax Gain Loss to be Reclassified within Twelve Months | (6.0)gmt_CashFlowHedgePreTaxGainLossToBeReclassifiedWithinTwelveMonths |
Expected After tax reclassification of net losses from accumulated other comprehensive income to earnings in Next Twelve Months | 3.8 us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths |
Aggregate fair value of all derivative instruments with Net liability position | 5.9 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue |
Derivative Asset, Notional Amount | $ 600.0 us-gaap_DerivativeAssetNotionalAmount | $ 200.0 us-gaap_DerivativeAssetNotionalAmount
Fair Value Hedging [Member] |  |
Fair Value Disclosure (Textual) [Abstract] |  |
Number of instruments, outstanding | 8 us-gaap_DerivativeNumberOfInstrumentsHeld / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_FairValueHedgingMember | 3 us-gaap_DerivativeNumberOfInstrumentsHeld / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_FairValueHedgingMember
Cash Flow Hedges [Member] |  |
Fair Value Disclosure (Textual) [Abstract] |  |
Number of instruments, outstanding | 7 us-gaap_DerivativeNumberOfInstrumentsHeld / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_CashFlowHedgingMember | 6 us-gaap_DerivativeNumberOfInstrumentsHeld / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = us-gaap_CashFlowHedgingMember

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- Definition Cash flow hedge pre-tax gain (loss) to be reclassified within twelve months. + References No definition available. + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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- Definition Nominal or face amount used to calculate payments on the derivative asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 + Details
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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
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- Definition The number of derivative instruments of a particular group held by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959 + Details
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