Debt instrument prepayment premium.

No definition available.

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Namespace Prefix:	rock_
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Balance Type:	debit
Period Type:	duration

- Definition

Debt instrument redemption percentage of aggregate principal amount.

+ References

No definition available.

+ Details

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- Definition

Dividend payment restrictions, aggregate annual limit.

+ References

No definition available.

+ Details

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- Definition

Dividend payment restrictions, annual limits per share.

+ References

No definition available.

+ Details

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- Definition

Minimum Interest Coverage Ratio

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Purchase Price Of The Principal Amount, Percentage

+ References

No definition available.

+ Details

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- Definition

Revolving credit facility minimum fixed charge coverage ratio.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
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+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

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

+ 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 210.5-02.22(a)(1))
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-Name Regulation S-X (SX)
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-Section 02
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- 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

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Balance Type:	na
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- 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

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Period Type:	duration

- Definition

Percentage of principal amount of debt redeemed.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

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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
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-Paragraph 1A
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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

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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

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
-Paragraph 1
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-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19, 20, 22
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+ Details

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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

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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))
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-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19, 22
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- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-Number 210
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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-Subparagraph (SX 210.5-02.19(b),22(b))
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-Name Regulation S-X (SX)
-Number 210
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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
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+ Details

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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
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

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- 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

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Debt (Narrative) (Details) - USD ($)		1 Months Ended		3 Months Ended	12 Months Ended
Debt (Narrative) (Details) - USD ($)	Dec. 09, 2015	Jan. 31, 2014	Jan. 31, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Standby letters of credit					$ 20,459,000
Availability amount					279,541,000
Credit facility amount outstanding					0	$ 0
Fixed charge coverage ratio	450.00%
Paid placement and other fees					$ 1,166,000	$ 35,000	$ 3,899,000
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]
Fixed charge coverage ratio	325.00%
Minimum interest coverage ratio	300.00%
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				1,265,000
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%
Senior Subordinated 6.25% Notes \| Minimum \| 12-Month Redemption Period Beginning February 1, 2018
Debt Instrument [Line Items]
Redemption price percentage		101.56%
Senior Subordinated 6.25% Notes \| Maximum \| 12-Month Redemption Period Beginning February 1, 2017
Debt Instrument [Line Items]
Redemption price percentage		103.13%
Revolving Credit Facility \| Senior Credit Agreement
Debt Instrument [Line Items]
Current borrowing capacity	$ 300,000,000
Maximum borrowing capacity under the revolving credit facility	$ 500,000,000
Revolving Credit Facility \| Senior Credit Agreement \| Minimum
Debt Instrument [Line Items]
Annual commitment fee	0.20%
Revolving Credit Facility \| Senior Credit Agreement \| Minimum \| LIBOR
Debt Instrument [Line Items]
LIBOR floor, plus	1.25%
Revolving Credit Facility \| Senior Credit Agreement \| Maximum
Debt Instrument [Line Items]
Annual commitment fee	0.30%
Revolving Credit Facility \| Senior Credit Agreement \| Maximum \| LIBOR
Debt Instrument [Line Items]
LIBOR floor, plus	2.25%
Term Loan \| Senior Credit Agreement
Debt Instrument [Line Items]
Term loan (up to)	$ 200,000,000