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Acquisitions (Allocation of Purchase Consideration to the Assets Acquired and Liabilities Assumed) (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Business Acquisition [Line Items]      
Goodwill $ 235,523,000us-gaap_Goodwill $ 236,044,000us-gaap_Goodwill  
September 2013 Acquisition      
Business Acquisition [Line Items]      
Working capital     2,665,000rock_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedWorkingCapital
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
Property, plant, and equipment     153,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
Acquired intangible assets     2,170,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
Goodwill 2,466,000us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
  2,466,000us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
Fair value of purchase consideration     $ 7,454,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember