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Goodwill and Related Intangible Assets (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 reporting_unit | Dec. 31, 2013 reporting_unit | Dec. 31, 2012 | Sep. 30, 2013 | Oct. 31, 2014 | Oct. 31, 2013
Goodwill [Line Items] |  |  |  |  |  |
Accumulated impairment losses | $ 255,530 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | $ 150,965 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss |  |  |  |
Goodwill impairment | (104,565)us-gaap_GoodwillImpairmentLoss | (21,040)us-gaap_GoodwillImpairmentLoss | (4,328)us-gaap_GoodwillImpairmentLoss |  |  |
Number of reporting units tested |  | 4 rock_NumberofReportingUnitsTestedforImpairment |  |  |  |
Number of reporting units | 11 us-gaap_NumberOfReportingUnits |  |  |  |  |
Industrial and Infrastructure Products |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
Number of reporting units tested |  | 1 rock_NumberofReportingUnitsTestedforImpairment / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember |  |  |  |
Number of reporting units |  | 2 us-gaap_NumberOfReportingUnits / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialAndInfrastructureProductsMember |  |  |  |
Residential Products |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
Number of reporting units |  | 2 us-gaap_NumberOfReportingUnits / us-gaap_StatementBusinessSegmentsAxis = rock_ResidentialProductsMember |  |  |  |
Industrial Specialty Products |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
Goodwill impairment |  |  |  | $ (21,040)us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = rock_IndustrialSpecialtyProductsMember |  |
Maximum |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
WACC |  |  |  |  | 13.60%rock_WeightedAverageCostOfCapital / us-gaap_RangeAxis = us-gaap_MaximumMember | 12.70%rock_WeightedAverageCostOfCapital / us-gaap_RangeAxis = us-gaap_MaximumMember
Minimum |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
WACC |  |  |  |  | 12.90%rock_WeightedAverageCostOfCapital / us-gaap_RangeAxis = us-gaap_MinimumMember | 11.10%rock_WeightedAverageCostOfCapital / us-gaap_RangeAxis = us-gaap_MinimumMember

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- Definition Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a,h) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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