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Pension and Other Post-Retirement Benefits (Details 10) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Multiemployer Plans |  |  |  |  |  |
Contributions | $ 8.5 us-gaap_MultiemployerPlanPeriodContributions |  | $ 8.1 us-gaap_MultiemployerPlanPeriodContributions |  | $ 8.1 us-gaap_MultiemployerPlanPeriodContributions |
American Maritime Officers Pension Plan [Member] |  |  |  |  |  |
Multiemployer Plans |  |  |  |  |  |
Zone Status | Yellow |  |  |  |  |
Contributions | 1.5 us-gaap_MultiemployerPlanPeriodContributions / gmt_MultiemployerPlansAxis = gmt_AmericanMaritimeOfficersPensionPlanMember | [1] | 1.4 us-gaap_MultiemployerPlanPeriodContributions / gmt_MultiemployerPlansAxis = gmt_AmericanMaritimeOfficersPensionPlanMember | [1] | 1.5 us-gaap_MultiemployerPlanPeriodContributions / gmt_MultiemployerPlansAxis = gmt_AmericanMaritimeOfficersPensionPlanMember | [1]
Collective Bargaining | Jan. 15, 2017 |  |  |  |  |
Other Multiemployer Post Retirement Plans [Member] |  |  |  |  |  |
Multiemployer Plans |  |  |  |  |  |
Contributions | $ 7.0 us-gaap_MultiemployerPlanPeriodContributions / gmt_MultiemployerPlansAxis = gmt_OtherMultiemployerPostRetirementPlansMember |  | $ 6.7 us-gaap_MultiemployerPlanPeriodContributions / gmt_MultiemployerPlansAxis = gmt_OtherMultiemployerPostRetirementPlansMember |  | $ 6.6 us-gaap_MultiemployerPlanPeriodContributions / gmt_MultiemployerPlansAxis = gmt_OtherMultiemployerPostRetirementPlansMember |
[1] | (1)Our contributions represented more than 5% of the total contributions to the plan during each year and no surcharge was imposed for any year. The actuary for the American Maritime Officers Pension Plan certified that the plan is in endangered status (i.e. “yellow zone” as defined by the Pension Protection Act of 2006) for the plan year beginning October 1, 2013, because it has funding or liquidity problems, or both. A rehabilitation plan, as defined by the Employee Retirement Security Act of 1974, was instituted under which certain adjustable benefits were reduced or eliminated, and we are required to contribute at a negotiated rate per day worked by each employee.

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- Definition Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450702-114947 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450691-114947 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Indicate the multiemployer plan's certified zone status as currently required by the Pension Protection Act of 2006 or a subsequent amendment thereof. The zone status categories are red (critical status), orange (seriously endangered), yellow (endangered), and green (none of the above). The acceptable values are "red", "orange", "yellow", "green", "other", and "NA". A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Definition Expiration date of the collective bargaining arrangement requiring contributions to the multiemployer plan, in CCYY-MM-DD format. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |