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Acquisitions (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 1 Months Ended | 2 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 business | Dec. 31, 2012 | Sep. 30, 2013 | Dec. 31, 2012 business
Business Acquisition [Line Items] |  |  |  |  |
Number of businesses acquired |  | 1 us-gaap_NumberOfBusinessesAcquired |  |  |
Legal and consulting fees | $ (1,388)us-gaap_BusinessCombinationAcquisitionRelatedCosts | $ 651 us-gaap_BusinessCombinationAcquisitionRelatedCosts | $ 700 us-gaap_BusinessCombinationAcquisitionRelatedCosts |  |
Goodwill | 236,044 us-gaap_Goodwill | 341,174 us-gaap_Goodwill | 359,863 us-gaap_Goodwill |  | 359,863 us-gaap_Goodwill
SG&A |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Legal and consulting fees | (1,594)us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember | (34)us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember | 456 us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember |  |
2013 Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Fair value of purchase consideration |  | 7,454 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member |  | 7,454 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member |
Payment for aggregate purchase consideration | 328 us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member | 1,864 us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member |  | 2,322 us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member |
Goodwill |  | 2,466 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member |  |  |
2013 Acquisitions | SG&A |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Adjustment to payable, expense recorded | (1,611)us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsExpensesAndLossesRecognized / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember | (236)us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsExpensesAndLossesRecognized / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember |  |  |
2013 Acquisitions | Interest Expense |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Adjustment to payable, expense recorded | 75 us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsExpensesAndLossesRecognized / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_IncomeStatementLocationAxis = us-gaap_InterestExpenseMember | 36 us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsExpensesAndLossesRecognized / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2013Member / us-gaap_IncomeStatementLocationAxis = us-gaap_InterestExpenseMember |  |  |
2012 Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Fair value of purchase consideration |  |  | 43,263 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member |  | 43,263 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member
Number of businesses acquired |  |  | 4 us-gaap_NumberOfBusinessesAcquired / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member |  | 3 us-gaap_NumberOfBusinessesAcquired / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member
Number of key markets |  |  | 4 rock_NumberOfKeyMarkets / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member |  |
Goodwill |  |  | 15,263 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member |  | 15,263 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member
2012 Acquisitions | Purchased Assets of Four Businesses |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Fair value of purchase consideration |  | $ 146 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = rock_AcquisitionsDuring2012Member / us-gaap_PropertyPlantAndEquipmentByTypeAxis = rock_PurchasedAssetsOfFourBusinessesMember |  |  |

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e1043-128460 + Details
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- Definition Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6405-128476 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
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- Definition Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 55 -Paragraph 37 -URI http://asc.fasb.org/extlink&oid=35744172&loc=d3e2207-128464 + Details
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- Definition The expenses and losses recorded for each transaction with the acquiree that was recognized separately from the acquisition of assets and assumptions of liabilities in the business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 + Details
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