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Supplemental Financial Information (Narrative) (Details)	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2013
Senior Subordinated 8% Notes, Due December 1, 2015
Debt Instrument [Line Items]
Senior Subordinated Notes, interest rate			8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedEightPercentNotesDueDecemberOneTwoThousandFifteenMember
Senior Subordinated 6.25% Notes
Debt Instrument [Line Items]
Senior Subordinated Notes, interest rate	6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember	6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember	6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember

X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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

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