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