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Income Taxes (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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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 | $ 10,014 us-gaap_DeferredTaxAssetsNetCurrent | $ 7,622 us-gaap_DeferredTaxAssetsNetCurrent |
Undistributed earnings of foreign subsidiaries | 28,237 us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings |  |
Cash held in foreign operations | 19,144 rock_CashHeldInForeignOperations |  |
Deferred tax assets, net of operating losses | 1,796 us-gaap_DeferredTaxAssetsOperatingLossCarryforwards |  |
Deferred tax assets, net of operating losses | $ 31,700 us-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 |  |

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- Definition Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards expected to be realized or consumed within one year or operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31917-109318 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31931-109318 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31928-109318 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31958-109318 + Details
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- Definition Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 + Details
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