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Summary of Significant Accounting Policies (Summary of Activity Recorded within the Allowance for Doubtful Accounts) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 4,774us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent	$ 4,481us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent	$ 4,614us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
Bad debt expense	1,095us-gaap_ProvisionForDoubtfulAccounts	910us-gaap_ProvisionForDoubtfulAccounts	920us-gaap_ProvisionForDoubtfulAccounts
Reserves from acquisitions	0rock_ReservesFromAcquisitions	183rock_ReservesFromAcquisitions	189rock_ReservesFromAcquisitions
Accounts written off	(1,589)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs	(800)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs	(1,242)us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Ending balance	$ 4,280us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent	$ 4,774us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent	$ 4,481us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent

X

- Definition

Reserves from acquisitions

+ References

No definition available.

+ Details

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Namespace Prefix:	rock_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 4

-Article 5



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.4)

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

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Namespace Prefix:	us-gaap_
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- Details

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

X

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of expense related to write-down of receivables to the amount expected to be collected. Includes, but is not limited to, accounts receivable and notes receivable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.5)

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



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 5

-Article 5

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration