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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
	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	$ 1,616us-gaap_AssetImpairmentCharges	$ 2,072us-gaap_AssetImpairmentCharges
Corporate
Restructuring Cost and Reserve [Line Items]
Total asset impairment charges (recovery) related to restructuring activities	0us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_CorporateNonSegmentMember	0us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_CorporateNonSegmentMember	140us-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	1,616us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember	1,932us-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	$ 125us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember	$ 0us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember	$ 0us-gaap_AssetImpairmentCharges / us-gaap_ConsolidationItemsAxis = us-gaap_OperatingSegmentsMember / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember

X

- 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

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