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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 $ 5,570us-gaap_InventoryValuationReserves $ 4,907us-gaap_InventoryValuationReserves $ 4,146us-gaap_InventoryValuationReserves
Excess, obsolete and slow moving inventory expense 731rock_InventoryValuationExcessObsoleteAndSlowMovingInventory 2,068rock_InventoryValuationExcessObsoleteAndSlowMovingInventory 2,417rock_InventoryValuationExcessObsoleteAndSlowMovingInventory
Reserves from acquisitions 0rock_ReservesFromAcquisitionsInventory 35rock_ReservesFromAcquisitionsInventory 95rock_ReservesFromAcquisitionsInventory
Scrapped inventory and other adjustments (726)rock_InventoryValuationScrappedInventoryAndOtherAdjustments (1,440)rock_InventoryValuationScrappedInventoryAndOtherAdjustments (1,751)rock_InventoryValuationScrappedInventoryAndOtherAdjustments
Ending balance $ 5,575us-gaap_InventoryValuationReserves $ 5,570us-gaap_InventoryValuationReserves $ 4,907us-gaap_InventoryValuationReserves