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Goodwill and Related Intangible Assets (Schedule of Acquired Intangible Assets) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Finite-lived intangible assets | 84,524 us-gaap_FiniteLivedIntangibleAssetsGross | $ 86,117 us-gaap_FiniteLivedIntangibleAssetsGross
Total acquired intangible assets, Gross Carrying Amount | 127,244 rock_AcquiredIntangibleAssetsTotal | 131,841 rock_AcquiredIntangibleAssetsTotal
Accumulated Amortization, Finite-lived intangible assets | 45,029 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization | 40,064 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Total acquired intangible assets, Accumulated Amortization | 45,029 rock_TotalAcquiredIntangibleAssetsAccumulatedAmortization | 40,064 rock_TotalAcquiredIntangibleAssetsAccumulatedAmortization
Trademarks |  |
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Finite-lived intangible assets | 3,886 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | 3,989 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
Accumulated Amortization, Finite-lived intangible assets | 1,827 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | 1,420 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
Unpatented technology and patents |  |
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Finite-lived intangible assets | 24,527 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UnpatentedTechnologyMember | 24,690 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UnpatentedTechnologyMember
Accumulated Amortization, Finite-lived intangible assets | 8,768 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UnpatentedTechnologyMember | 6,980 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_UnpatentedTechnologyMember
Customer relationships |  |
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Finite-lived intangible assets | 52,974 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 54,171 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Accumulated Amortization, Finite-lived intangible assets | 31,554 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember | 28,926 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember
Non-compete agreements |  |
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Finite-lived intangible assets | 1,807 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember | 1,937 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember
Accumulated Amortization, Finite-lived intangible assets | 1,550 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember | 1,408 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_NoncompeteAgreementsMember
Backlog |  |
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Finite-lived intangible assets | 1,330 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OrderOrProductionBacklogMember | 1,330 us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OrderOrProductionBacklogMember
Accumulated Amortization, Finite-lived intangible assets | 1,330 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OrderOrProductionBacklogMember | 1,330 us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OrderOrProductionBacklogMember
Trademarks |  |
Finite-Lived Intangible Assets [Line Items] |  |
Gross Carrying Amount, Indefinite-lived intangible assets | 42,720 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | 45,724 us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
Accumulated Amortization, Indefinite-lived intangible assets | 0 rock_IndefiniteLivedAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember | $ 0 rock_IndefiniteLivedAccumulatedAmortization / us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TrademarksMember
Minimum | Trademarks |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 2 years |
Minimum | Unpatented technology and patents |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 5 years |
Minimum | Customer relationships |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 5 years |
Minimum | Non-compete agreements |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 4 years |
Minimum | Backlog |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 1 year |
Maximum | Trademarks |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 15 years |
Maximum | Unpatented technology and patents |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 20 years |
Maximum | Customer relationships |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 16 years |
Maximum | Non-compete agreements |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 10 years |
Maximum | Backlog |  |
Finite-Lived Intangible Assets [Line Items] |  |
Estimated Useful Life | 2 years |

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