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Exit Activity Costs and Asset Impairments (Schedule Of Asset Impairment Charges Related To Restructuring Activities) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Restructuring Cost and Reserve [Line Items] |  |  |
Total asset impairment charges (recovery) related to restructuring activities | $ (455)us-gaap_AssetImpairmentCharges | $ 1,616 us-gaap_AssetImpairmentCharges | $ 2,072 us-gaap_AssetImpairmentCharges
Corporate |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Total asset impairment charges (recovery) related to restructuring activities | 0 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_CorporateNonSegmentMember | 0 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_CorporateNonSegmentMember | 140 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_CorporateNonSegmentMember
Residential Products | Operating Segments |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Total asset impairment charges (recovery) related to restructuring activities | (580)us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember | 1,616 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember | 1,932 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember
Industrial and Infrastructure Products | Operating Segments |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Total asset impairment charges (recovery) related to restructuring activities | $ 125 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember | $ 0 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember | $ 0 us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember

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- Definition Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill. + 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 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228 + Details
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