XML 84 R72.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Supplemental Financial Information (Narrative) (Details) | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017 | Mar. 31, 2013
Schedule Of Supplemental Financial Information [Line Items] |  |  |
Ownership percentage | 100.00% |  |
Senior Subordinated 6.25% Notes |  |  |
Schedule Of Supplemental Financial Information [Line Items] |  |  |
Senior Subordinated Notes, interest rate | 6.25% | 6.25% | 6.25%

X
- Definition Ownership percentage of subsidiary guarantor by parent company. + References No definition available. + Details
Name: | rock_OwnershipPercentageOfSubsidiaryGuarantorByParentCompany
Namespace Prefix: | rock_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Schedule Of Supplemental Financial Information [Line Items] + References No definition available. + Details
Name: | rock_ScheduleOfSupplementalFinancialInformationLineItems
Namespace Prefix: | rock_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DebtInstrumentAxis=rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |