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Exit Activity Costs and Asset Impairments (Summary of Exit Activity Costs and Asset Impairments) (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
Cost of sales    
Restructuring Cost and Reserve [Line Items]    
Net asset impairment and exit activity (gains) charges 188us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_CostOfSalesMember
325us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_CostOfSalesMember
Selling, general, and administrative expense    
Restructuring Cost and Reserve [Line Items]    
Net asset impairment and exit activity (gains) charges $ (6,768)us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= rock_SellingGeneralAndAdministrativeExpenseMember
$ 104us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= rock_SellingGeneralAndAdministrativeExpenseMember