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Accumulated Other Comprehensive (Loss) Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Balance at begging of period	$ (4,381)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1	$ (2,429)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
Other comprehensive loss on cash flow hedge before reclassification	(875)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
Reclassified loss on cash flow hedge from other comprehensive (loss) income	650us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
Balance at end of period	(11,268)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1	(4,381)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1	(2,429)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
Tax (Benefit) Expense
Balance at beginning of period	(796)rock_AccumulatedOtherComprehensiveIncomeLossTax	(854)rock_AccumulatedOtherComprehensiveIncomeLossTax
Other comprehensive loss on cash flow hedge before reclassification	(319)rock_OtherComprehensiveIncomeLossbeforeReclassificationsTax
Reclassified loss on cash flow hedge from other comprehensive (loss) income	237rock_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
Minimum pension and post retirement health care plan adjustments	(839)us-gaap_OtherComprehensiveIncomeLossTax	58us-gaap_OtherComprehensiveIncomeLossTax
Balance at end of period	(1,717)rock_AccumulatedOtherComprehensiveIncomeLossTax	(796)rock_AccumulatedOtherComprehensiveIncomeLossTax	(854)rock_AccumulatedOtherComprehensiveIncomeLossTax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at beginning of period	(3,585)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax	(1,575)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Other comprehensive loss on cash flow hedge before reclassification	(556)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax
Reclassified loss on cash flow hedge from other comprehensive (loss) income	413us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
Other Comprehensive Income (Loss), Net of Tax	(5,966)us-gaap_OtherComprehensiveIncomeLossNetOfTax	(2,010)us-gaap_OtherComprehensiveIncomeLossNetOfTax	1,775us-gaap_OtherComprehensiveIncomeLossNetOfTax
Balance at end of period	(9,551)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax	(3,585)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax	(1,575)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Balance at begging of period	(2,201)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember	(93)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Other Comprehensive Income (Loss), before Tax	(4,364)us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember	(2,108)us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Balance at end of period	(6,565)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember	(2,201)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other Comprehensive Income (Loss), Net of Tax	(4,364)us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember	(2,108)us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Balance at begging of period	0us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Other comprehensive loss on cash flow hedge before reclassification	(875)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Reclassified loss on cash flow hedge from other comprehensive (loss) income	650us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Balance at end of period	(225)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		0us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Unrealized loss on cash flow hedges expected to be reclassified into earnings within twelve months	225us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Other Comprehensive Income (Loss), before Tax	(2,298)us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	156us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other Comprehensive Income (Loss), Net of Tax	(1,459)us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	98us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Accumulated Defined Benefit Plans Adjustment | Minimum Pension Liability Adjustment
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Balance at begging of period	76us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	(8)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Other Comprehensive Income (Loss), before Tax	(33)us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	84us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Balance at end of period	43us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	76us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Accumulated Defined Benefit Plans Adjustment | Unamortized Post-Retirement Health Care Costs
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Balance at begging of period	(2,256)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	(2,328)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Other Comprehensive Income (Loss), before Tax	(2,265)us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	72us-gaap_OtherComprehensiveIncomeLossBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Balance at end of period	$ (4,521)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember	$ (2,256)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember

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Amount before tax of increase (decrease) in accumulated in equity from transactions and other events and circumstances from non-owner sources. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners).

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Amount before tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount before tax, after reclassification adjustments of other comprehensive income (loss).

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Amount of tax expense (benefit) allocated to other comprehensive income (loss).

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