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Fair Value Measurements (Fair Value of Assets (Liabilities)) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Accrued expenses | Contingent consideration liability |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | $ 328 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember | $ 1,864 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember
Accrued expenses | Contingent consideration liability | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Accrued expenses | Contingent consideration liability | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Accrued expenses | Contingent consideration liability | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 328 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 1,864 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_AccruedLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = rock_ContingentConsiderationMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Other current assets | Foreign exchange options |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 1,851 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember |
Other current assets | Foreign exchange options | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Other current assets | Foreign exchange options | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 1,851 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Other current assets | Foreign exchange options | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Other current assets | Commodity options |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 591 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember |
Other current assets | Commodity options | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Other current assets | Commodity options | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 591 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Other current assets | Commodity options | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Other assets | Foreign exchange options |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 445 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember |
Other assets | Foreign exchange options | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Other assets | Foreign exchange options | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 445 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Other assets | Foreign exchange options | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Other assets | Commodity options |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 162 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember |
Other assets | Commodity options | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Other assets | Commodity options | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 162 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Other assets | Commodity options | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityOptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Long-term debt | Total long-term debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 215,831 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember | 220,825 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember
Long-term debt | Total long-term debt | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 215,831 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 220,825 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Long-term debt | Total long-term debt | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Long-term debt | Total long-term debt | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Net Asset (Liability) | $ 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | $ 0 us-gaap_FairValueNetAssetLiability / us-gaap_BalanceSheetLocationAxis = us-gaap_LongTermDebtMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_SeniorSubordinatedNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member

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