XML 97 R79.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Employee Retirement Plans (Schedule of Contributions Made by the Company to Each Multiemployer Plan) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | $ 292 | $ 296 | $ 333
National Integrated Group Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | 220 | 218 | 246
Sheet Metal Workers National Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | 42 | 50 | 56
Sheet Metal Workers Pension Plan Of Northern California |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | $ 30 | $ 28 | $ 31

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of contributions made to multiemployer plan by employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450702-114947 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450691-114947 + Details
Name: | us-gaap_MultiemployerPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_MultiemployerPlanNameAxis=rock_NationalIntegratedGroupPensionPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MultiemployerPlanNameAxis=rock_SheetMetalWorkersNationalPensionPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MultiemployerPlanNameAxis=rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |