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Consolidated Statements of Shareholders' Equity (USD $)
In Thousands, except Share data, unless otherwise specified
Total
Common Stock
Additional Paid-In Capital
Retained Earnings
Accumulated Other Comprehensive Loss
Treasury Stock
Balance at Dec. 31, 2011 $ 459,936us-gaap_StockholdersEquity $ 307us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
$ 236,673us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
$ 229,437us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
$ (3,350)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
$ (3,131)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Balance, shares at Dec. 31, 2011   30,702,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      281,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Net (loss) income 12,645us-gaap_NetIncomeLoss     12,645us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
   
Foreign currency translation adjustment 2,353us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax       2,353us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Adjustment to retirement benefit liability, net of tax (79)rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentNetOfTax1       (79)rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentNetOfTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Adjustment to post-retirement healthcare benefit liability, net of taxes (499)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax       (499)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Unrealized loss on cash flow hedges, net of tax 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax          
Stock compensation expense 3,148us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue   3,148us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Excess tax benefit from stock compensation 10us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation   10us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Net settlement of restricted stock units, shares   197,000rock_StockIssuedDuringPeriodSharesRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      69,000rock_StockIssuedDuringPeriodSharesRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Net settlement of restricted stock units (970)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement 2rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
(2)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
    (970)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Issuance of restricted stock, shares   11,000us-gaap_StockIssuedDuringPeriodSharesOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Stock options exercised, shares 27,500us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised 28,000us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Stock options exercised 278us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised   278us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Balance at Dec. 31, 2012 476,822us-gaap_StockholdersEquity 309us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
240,107us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
242,082us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(1,575)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(4,101)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Balance, shares at Dec. 31, 2012   30,938,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      350,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Net (loss) income (5,633)us-gaap_NetIncomeLoss     (5,633)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
   
Foreign currency translation adjustment (2,108)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax       (2,108)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Adjustment to retirement benefit liability, net of tax 53rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentNetOfTax1       53rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentNetOfTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Adjustment to post-retirement healthcare benefit liability, net of taxes 45us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax       45us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Unrealized loss on cash flow hedges, net of tax 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax          
Stock compensation expense 2,564us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue   2,564us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Excess tax benefit from stock compensation 72us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation   72us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Net settlement of restricted stock units, shares   120,000rock_StockIssuedDuringPeriodSharesRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      45,000rock_StockIssuedDuringPeriodSharesRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Net settlement of restricted stock units (714)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement 2rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
(2)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
    (714)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Issuance of restricted stock, shares   13,000us-gaap_StockIssuedDuringPeriodSharesOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Stock options exercised, shares 59,750us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised 60,000us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Stock options exercised 648us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised   648us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Balance at Dec. 31, 2013 471,749us-gaap_StockholdersEquity 311us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
243,389us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
236,449us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
(3,585)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
(4,815)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Balance, shares at Dec. 31, 2013   31,131,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      395,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Net (loss) income (81,824)us-gaap_NetIncomeLoss     (81,824)us-gaap_NetIncomeLoss
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
   
Foreign currency translation adjustment (4,364)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax       (4,364)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Adjustment to retirement benefit liability, net of tax (24)rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentNetOfTax1       (24)rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentNetOfTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Adjustment to post-retirement healthcare benefit liability, net of taxes (1,435)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax       (1,435)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Unrealized loss on cash flow hedges, net of tax (143)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax       (143)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
 
Stock compensation expense 3,150us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue   3,150us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Excess tax benefit from stock compensation 100us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation   100us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Net settlement of restricted stock units, shares   136,000rock_StockIssuedDuringPeriodSharesRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      34,000rock_StockIssuedDuringPeriodSharesRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Net settlement of restricted stock units (575)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement 1rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
(1)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
    (575)rock_StockIssuedDuringPeriodValueRestrictedStockAwardSettlement
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Issuance of restricted stock, shares   22,000us-gaap_StockIssuedDuringPeriodSharesOther
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Stock options exercised, shares 52,805us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised 53,000us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Stock options exercised 595us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised 1us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
594us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
     
Balance at Dec. 31, 2014 $ 387,229us-gaap_StockholdersEquity $ 313us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
$ 247,232us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AdditionalPaidInCapitalMember
$ 154,625us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember
$ (9,551)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
$ (5,390)us-gaap_StockholdersEquity
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember
Balance, shares at Dec. 31, 2014   31,342,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
      429,000us-gaap_SharesIssued
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_TreasuryStockMember