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Acquisitions (Schedule of Acquired Intangible Assets) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012
2013 Acquisitions |  |
Business Acquisition [Line Items] |  |
Fair Value | $ 2,170 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member |
2013 Acquisitions | Trademarks |  |
Business Acquisition [Line Items] |  |
Fair Value | 640 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember |
2013 Acquisitions | Customer relationships |  |
Business Acquisition [Line Items] |  |
Fair Value | 1,130 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
Estimated Useful Life | 15 years |
2013 Acquisitions | Technology |  |
Business Acquisition [Line Items] |  |
Fair Value | 260 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TechnologyEquipmentMember |
Estimated Useful Life | 15 years |
2013 Acquisitions | Non-compete agreements |  |
Business Acquisition [Line Items] |  |
Fair Value | 140 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember |
Estimated Useful Life | 5 years |
2012 Acquisitions |  |
Business Acquisition [Line Items] |  |
Fair Value |  | 10,183 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member
2012 Acquisitions | Trademarks |  |
Business Acquisition [Line Items] |  |
Fair Value |  | 2,130 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
2012 Acquisitions | Customer relationships |  |
Business Acquisition [Line Items] |  |
Fair Value |  | 4,470 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
2012 Acquisitions | Unpatented technology and patents |  |
Business Acquisition [Line Items] |  |
Fair Value |  | 2,313 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UnpatentedTechnologyMember
Estimated Useful Life |  | 15 years
2012 Acquisitions | Trademarks |  |
Business Acquisition [Line Items] |  |
Fair Value |  | 800 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
Estimated Useful Life |  | 5 years
2012 Acquisitions | Non-compete agreements |  |
Business Acquisition [Line Items] |  |
Fair Value |  | 340 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember
2012 Acquisitions | Backlog |  |
Business Acquisition [Line Items] |  |
Fair Value |  | $ 130 us-gaap_FinitelivedIntangibleAssetsAcquired1 / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OrderOrProductionBacklogMember
Estimated Useful Life |  | 0 years 6 months
2012 Acquisitions | Maximum | Customer relationships |  |
Business Acquisition [Line Items] |  |
Estimated Useful Life | 15 years |
2012 Acquisitions | Maximum | Non-compete agreements |  |
Business Acquisition [Line Items] |  |
Estimated Useful Life | 10 years |
2012 Acquisitions | Minimum | Customer relationships |  |
Business Acquisition [Line Items] |  |
Estimated Useful Life | 5 years |
2012 Acquisitions | Minimum | Non-compete agreements |  |
Business Acquisition [Line Items] |  |
Estimated Useful Life | 5 years |

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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16265-109275 + Details
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- Definition Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, resulting from a business combination. + References No definition available. + Details
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