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Goodwill and Related Intangible Assets (Schedule of Changes in Carrying Amount of Goodwill) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Goodwill [Roll Forward] |  |  |
Balance at | $ 341,174 us-gaap_Goodwill | $ 359,863 us-gaap_Goodwill |
Acquired goodwill |  | 2,467 us-gaap_GoodwillAcquiredDuringPeriod |
Adjustments to prior year acquisitions |  | 252 us-gaap_GoodwillPurchaseAccountingAdjustments |
Impairment | (104,565)us-gaap_GoodwillImpairmentLoss | (21,040)us-gaap_GoodwillImpairmentLoss | (4,328)us-gaap_GoodwillImpairmentLoss
Foreign currency translation | (565)us-gaap_GoodwillTranslationAdjustments | (368)us-gaap_GoodwillTranslationAdjustments |
Balance at | 236,044 us-gaap_Goodwill | 341,174 us-gaap_Goodwill | 359,863 us-gaap_Goodwill
Residential Products |  |  |
Goodwill [Roll Forward] |  |  |
Balance at | 195,520 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember | 193,043 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember |
Acquired goodwill |  | 2,467 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember |
Adjustments to prior year acquisitions |  | 10 us-gaap_GoodwillPurchaseAccountingAdjustments / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember |
Impairment | (14,235)us-gaap_GoodwillImpairmentLoss / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember | 0 us-gaap_GoodwillImpairmentLoss / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember |
Foreign currency translation | 0 us-gaap_GoodwillTranslationAdjustments / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember | 0 us-gaap_GoodwillTranslationAdjustments / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember |
Balance at | 181,285 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember | 195,520 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_ResidentialProductsMember |
Industrial and Infrastructure Products |  |  |
Goodwill [Roll Forward] |  |  |
Balance at | 145,654 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember | 166,820 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember |
Acquired goodwill |  | 0 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember |
Adjustments to prior year acquisitions |  | 242 us-gaap_GoodwillPurchaseAccountingAdjustments / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember |
Impairment | (90,330)us-gaap_GoodwillImpairmentLoss / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember | (21,040)us-gaap_GoodwillImpairmentLoss / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember |
Foreign currency translation | (565)us-gaap_GoodwillTranslationAdjustments / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember | (368)us-gaap_GoodwillTranslationAdjustments / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember |
Balance at | $ 54,759 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember | $ 145,654 us-gaap_Goodwill / us-gaap_ProductOrServiceAxis = rock_IndustrialAndInfrastructureProductsMember |

X
- 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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- Definition Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Amount of increase (decrease) from foreign currency translation adjustments 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 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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