Debt instrument prepayment premium.

No definition available.

Name:	rock_DebtInstrumentPrepaymentPremium
Namespace Prefix:	rock_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Debt instrument redemption percentage of aggregate principal amount.

+ References

No definition available.

+ Details

Name:	rock_DebtInstrumentRedemptionPercentageOfAggregatePrincipalAmount
Namespace Prefix:	rock_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Dividend payment restrictions, aggregate annual limit.

+ References

No definition available.

+ Details

Name:	rock_DividendPaymentRestrictionsAggregateAnnualLimits
Namespace Prefix:	rock_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Dividend payment restrictions, annual limits per share.

+ References

No definition available.

+ Details

Name:	rock_DividendPaymentRestrictionsAnnualLimitsPerShare
Namespace Prefix:	rock_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase Price Of The Principal Amount, Percentage

+ References

No definition available.

+ Details

Name:	rock_PurchasePriceOfPrincipalAmountPercentage
Namespace Prefix:	rock_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Revolving credit facility minimum fixed charge coverage ratio.

+ References

No definition available.

+ Details

Name:	rock_RevolvingCreditFacilityMinimumFixedChargeCoverageRatio
Namespace Prefix:	rock_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 470

-Section 50

-Paragraph 3

-Subparagraph e

-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentageOfPrincipalAmountRedeemed
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentRepurchaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of gain (loss) from the difference between the repurchase price of a debt instrument initially issued by the entity and the net carrying amount of the debt at the time of its repurchase.

+ References

No definition available.

+ Details

Name:	us-gaap_GainLossOnRepurchaseOfDebtInstrument
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value of notes issued in noncash investing and financing activities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

+ Details

Name:	us-gaap_NotesIssued1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow for loan and debt issuance costs.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsOfFinancingCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.8)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_WriteOffOfDeferredDebtIssuanceCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=rock_SeniorCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
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- Details

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Debt (Narrative) (Details) (USD $)	1 Months Ended	12 Months Ended			3 Months Ended	1 Months Ended
Debt (Narrative) (Details) (USD $)	Jan. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Standby letters of credit		$ 20,377,000
Paid placement and other fees		35,000us-gaap_PaymentsOfFinancingCosts	3,899,000us-gaap_PaymentsOfFinancingCosts	18,000us-gaap_PaymentsOfFinancingCosts
Dividend payment restrictions, annual limit per share	$ 0.25
Dividend payment restrictions, aggregate annual limit	25,000,000
Senior Credit Agreement
Debt Instrument [Line Items]
Current borrowing capacity		200,000,000
Maximum borrowing capacity under the revolving credit facility		250,000,000
Fixed charge coverage ratio		125.00%
Senior Subordinated 8% Notes, Due December 1, 2015
Debt Instrument [Line Items]
Interest rate						8.00%
Repurchase amount						204,000,000
Repurchase charge					7,166,000
Prepayment premium paid					3,702,000
Write off of deferred financing costs					2,199,000
Unamortized original issue discount			1,298,000		1,265,000
Senior Subordinated 6.25% Notes
Debt Instrument [Line Items]
Notes issued						210,000,000
Interest rate		6.25%	6.25%			6.25%
Paid placement and other fees					3,755,000
Senior Subordinated 6.25% Notes \| Change in Control
Debt Instrument [Line Items]
Purchase price of the principal amount, percentage	101.00%
Senior Subordinated 6.25% Notes \| On and After February 1, 2019
Debt Instrument [Line Items]
Redemption price, percentage of principal amount	100.00%
Senior Subordinated 6.25% Notes \| Prior to February 1, 2016
Debt Instrument [Line Items]
Redemption price, percentage of principal amount	106.25%
Redemption percentage of aggregate principal amount	35.00%
Revolving Credit Facility
Debt Instrument [Line Items]
Annual commitment fee		0.375%
Availability amount		$ 98,350,000
Minimum \| Senior Subordinated 6.25% Notes \| 12-Month Redemption Period Beginning February 1, 2018
Debt Instrument [Line Items]
Redemption price percentage	101.56%
Minimum \| Revolving Credit Facility
Debt Instrument [Line Items]
LIBOR floor, plus		2.00%
Maximum \| Senior Subordinated 6.25% Notes \| 12-Month Redemption Period Beginning February 1, 2017
Debt Instrument [Line Items]
Redemption price percentage	103.13%
Maximum \| Revolving Credit Facility
Debt Instrument [Line Items]
LIBOR floor, plus		2.50%