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Income Taxes (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Taxes [Line Items]      
Statutory tax rate, percentage 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Net current deferred tax assets $ 10,014us-gaap_DeferredTaxAssetsNetCurrent $ 7,622us-gaap_DeferredTaxAssetsNetCurrent  
Undistributed earnings of foreign subsidiaries 28,237us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings    
Cash held in foreign operations 19,144rock_CashHeldInForeignOperations    
Deferred tax assets, net of operating losses 1,796us-gaap_DeferredTaxAssetsOperatingLossCarryforwards    
Deferred tax assets, net of operating losses $ 31,700us-gaap_OperatingLossCarryforwards    
Minimum      
Income Taxes [Line Items]      
Statute of limitations expiration period, in years 4 years    
Net operating loss carry forwards, expiration years Dec. 31, 2015    
Minimum | Germany      
Income Taxes [Line Items]      
Tax examination, years 2009    
Maximum      
Income Taxes [Line Items]      
Statute of limitations expiration period, in years 10 years    
Net operating loss carry forwards, expiration years Dec. 31, 2033    
Maximum | Germany      
Income Taxes [Line Items]      
Tax examination, years 2012