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Pension and Other Post-Retirement Benefits (Details 1) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets [Abstract] |  |
Projected benefit obligations | $ 502.7 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation | $ 67.8 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation
Fair value of plan assets | 456.9 gmt_DefinedBenefitPlanPensionPlanswithAccumulatedBenefitObligationsinExcessofPlanAssetsAggregateProjectedFairValueofPlanAssets | 40.0 gmt_DefinedBenefitPlanPensionPlanswithAccumulatedBenefitObligationsinExcessofPlanAssetsAggregateProjectedFairValueofPlanAssets
Pension plans with an accumulated benefit obligation in excess of plan assets |  |
Accumulated benefit obligations | 170.6 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation | 66.4 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation
Fair value of plan assets | $ 140.5 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateFairValueOfPlanAssets | $ 40.0 us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateFairValueOfPlanAssets

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- Definition Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Projected Fair Value of Plan Assets + References No definition available. + Details
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- Definition If aggregate disclosures are presented, the aggregate pension accumulated benefit obligation for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=34716465&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920 + Details
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- Definition If aggregate disclosures are presented, the aggregate fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=34716465&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920 + Details
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- Definition If aggregate disclosures are presented, the aggregate projected benefit obligation for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=34716465&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920 + Details
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