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Accumulated Other Comprehensive (Loss) Income (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Total Pre-Tax Amount    
Balance at $ (11,268)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1  
Reclassified loss on cash flow hedge from other comprehensive income (loss) 225us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax  
Balance at (14,779)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1  
Tax (Benefit) Expense    
Balance at (1,717)rock_AccumulatedOtherComprehensiveIncomeLossTax  
Reclassified loss on cash flow hedge from other comprehensive income (loss) 82rock_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax  
Balance at (1,610)rock_AccumulatedOtherComprehensiveIncomeLossTax  
Accumulated Other Comprehensive (Loss) Income    
Balance at (9,551)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax  
Reclassified loss on cash flow hedge from other comprehensive income (loss) 143us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax  
Other comprehensive loss (3,618)us-gaap_OtherComprehensiveIncomeLossNetOfTax (883)us-gaap_OtherComprehensiveIncomeLossNetOfTax
Balance at (13,169)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax  
Accumulated Translation Adjustment [Member]    
Total Pre-Tax Amount    
Balance at (6,565)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
 
Other Comprehensive Income (Loss), before Tax (3,800)us-gaap_OtherComprehensiveIncomeLossBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
 
Balance at (10,365)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
 
Accumulated Other Comprehensive (Loss) Income    
Other comprehensive loss (3,800)us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
 
Cash Flow Hedges    
Total Pre-Tax Amount    
Balance at (225)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
 
Reclassified loss on cash flow hedge from other comprehensive income (loss) 225us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
 
Balance at 0us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
 
Accumulated Defined Benefit Plans Adjustment [Member]    
Total Pre-Tax Amount    
Other Comprehensive Income (Loss), before Tax 64us-gaap_OtherComprehensiveIncomeLossBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Tax (Benefit) Expense    
Minimum pension and post retirement health care plan adjustments 25us-gaap_OtherComprehensiveIncomeLossTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Accumulated Other Comprehensive (Loss) Income    
Other comprehensive loss 39us-gaap_OtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Minimum Pension Liability Adjustment | Accumulated Defined Benefit Plans Adjustment [Member]    
Total Pre-Tax Amount    
Balance at 43us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Other Comprehensive Income (Loss), before Tax 4us-gaap_OtherComprehensiveIncomeLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Balance at 47us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Postretirement Health Coverage [Member] | Accumulated Defined Benefit Plans Adjustment [Member]    
Total Pre-Tax Amount    
Balance at (4,521)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_DefinedBenefitPostretirementHealthCoverageMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Other Comprehensive Income (Loss), before Tax 60us-gaap_OtherComprehensiveIncomeLossBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_DefinedBenefitPostretirementHealthCoverageMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
 
Balance at $ (4,461)us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_DefinedBenefitPostretirementHealthCoverageMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember