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Exit Activity Costs and Asset Impairments (Summary of Exit Activity Costs and Asset Impairments by Segment) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Restructuring Cost and Reserve [Line Items]    
Net asset impairment and exit activity (gains) charges $ (6,580)us-gaap_BusinessExitCosts1 $ 429us-gaap_BusinessExitCosts1
Residential Products    
Restructuring Cost and Reserve [Line Items]    
Net asset impairment and exit activity (gains) charges (6,580)us-gaap_BusinessExitCosts1
/ us-gaap_StatementBusinessSegmentsAxis
= rock_ResidentialProductsMember
327us-gaap_BusinessExitCosts1
/ us-gaap_StatementBusinessSegmentsAxis
= rock_ResidentialProductsMember
Industrial and Infrastructure Products    
Restructuring Cost and Reserve [Line Items]    
Net asset impairment and exit activity (gains) charges $ 0us-gaap_BusinessExitCosts1
/ us-gaap_StatementBusinessSegmentsAxis
= rock_IndustrialAndInfrastructureProductsMember
$ 102us-gaap_BusinessExitCosts1
/ us-gaap_StatementBusinessSegmentsAxis
= rock_IndustrialAndInfrastructureProductsMember