Pension and Other Post-Retirement Benefits (Details 4)	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Post Age 65	6.00%	7.50%
Medical Claims [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Pre Age 65	7.00%	7.50%
Review of historical returns
Rate to which the cost trend is expected to decline (the ultimate trend rate)	4.50%	5.00%
Year that rate reaches the ultimate trend rate	2024	2023
Prescription Drugs Claims [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Pre Age 65	10.00%	7.00%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Post Age 65	10.50%	7.00%
Review of historical returns
Rate to which the cost trend is expected to decline (the ultimate trend rate)	4.50%	5.00%
Year that rate reaches the ultimate trend rate	2024	2023

Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Post Age 65

No definition available.

Name:	gmt_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearPostAge65
Namespace Prefix:	gmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Pre Age 65

+ References

No definition available.

+ Details

Name:	gmt_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearPreAge65
Namespace Prefix:	gmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumedHealthCareCostTrendRatesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The ultimate trend rate for health care costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (l)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Year the ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanYearThatRateReachesUltimateTrendRate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=gmt_MedicalClaimsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=gmt_PrescriptionDrugsClaimsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: