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Employee Retirement Plans (Schedule of Contributions Made by the Company to Each Multiemployer Plan) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Contributions	$ 329us-gaap_MultiemployerPlanPeriodContributions	$ 328us-gaap_MultiemployerPlanPeriodContributions	$ 333us-gaap_MultiemployerPlanPeriodContributions
National Integrated Group Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions	233us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_NationalIntegratedGroupPensionPlanMember	222us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_NationalIntegratedGroupPensionPlanMember	202us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_NationalIntegratedGroupPensionPlanMember
Sheet Metal Workers National Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions	61us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersNationalPensionPlanMember	78us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersNationalPensionPlanMember	98us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersNationalPensionPlanMember
Sheet Metal Workers Pension Plan Of Northern California [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions	$ 35us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember	$ 28us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember	$ 33us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember

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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

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