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Inventories (Summary of Activity within the Reserve for Excess, Obsolete, and Slow Moving Inventory) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Inventory [Roll Forward] |  |  |
Beginning balance | $ 5,570 us-gaap_InventoryValuationReserves | $ 4,907 us-gaap_InventoryValuationReserves | $ 4,146 us-gaap_InventoryValuationReserves
Excess, obsolete and slow moving inventory expense | 731 rock_InventoryValuationExcessObsoleteAndSlowMovingInventory | 2,068 rock_InventoryValuationExcessObsoleteAndSlowMovingInventory | 2,417 rock_InventoryValuationExcessObsoleteAndSlowMovingInventory
Reserves from acquisitions | 0 rock_ReservesFromAcquisitionsInventory | 35 rock_ReservesFromAcquisitionsInventory | 95 rock_ReservesFromAcquisitionsInventory
Scrapped inventory and other adjustments | (726)rock_InventoryValuationScrappedInventoryAndOtherAdjustments | (1,440)rock_InventoryValuationScrappedInventoryAndOtherAdjustments | (1,751)rock_InventoryValuationScrappedInventoryAndOtherAdjustments
Ending balance | $ 5,575 us-gaap_InventoryValuationReserves | $ 5,570 us-gaap_InventoryValuationReserves | $ 4,907 us-gaap_InventoryValuationReserves

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- Definition Amount of valuation reserve for inventory. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 6 -Subparagraph a -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6386567&loc=d3e3927-108312 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.BB) -URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 09 -Article 12 + Details
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