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Exit Activity Costs and Asset Impairments (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 facility | Dec. 31, 2013 facility | Dec. 31, 2012 facility
Restructuring and Related Activities [Abstract] |  |  |
Number of consolidated facilities | 2 rock_NumberOfConsolidatedFacilities | 2 rock_NumberOfConsolidatedFacilities | 2 rock_NumberOfConsolidatedFacilities
Number of consolidated facilities to be closed | 1 rock_NumberofConsolidatedFacilitiestobeClosed |  |
Expected future restructuring costs | $ 455 us-gaap_RestructuringAndRelatedCostExpectedCost1 |  |

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- Definition Number of Consolidated Facilities to be Closed + References No definition available. + Details
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- Details
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- Definition Amount expected to be recognized in earnings for the specified restructuring cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section P -Subsection 3, 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 5.P.4(b)) -URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140904-122747 + Details
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