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Goodwill and Related Intangible Assets (Schedule of Impairment Charges) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Impairment charges	$ 3,405us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill	$ 2,454us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill	$ 300us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
Residential Products
Finite-Lived Intangible Assets [Line Items]
Impairment charges	1,200us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember	1,454us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember	300us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember
Industrial and Infrastructure Products
Finite-Lived Intangible Assets [Line Items]
Impairment charges	$ 2,205us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember	$ 1,000us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember	$ 0us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember

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- Definition

The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of an intangible asset (excluding goodwill) to fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 3

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16373-109275

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