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Goodwill And Related Intangible Assets (Schedule Of Goodwill Impairment Tests) (Details) | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2013 Maximum [Member] | Oct. 31, 2013 Maximum [Member] | Oct. 31, 2012 Maximum [Member] | Dec. 31, 2013 Minimum [Member] | Oct. 31, 2013 Minimum [Member] | Oct. 31, 2012 Minimum [Member]
Goodwill [Line Items] |  |  |  |  |  |  |  |
Date of impairment test | Oct. 31, 2013 | Oct. 31, 2012 |  |  |  |  |  |
WACC |  |  | 13.40% | 12.70% | 13.10% | 12.90% | 11.10% | 10.70%

X
- Definition Weighted Average Cost Of Capital + References No definition available. + Details
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Namespace Prefix: | rock_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Weighted average cost of capital used to discount cash flows in the annual goodwill impairment test. + References No definition available. + Details
Name: | rock_WeightedAverageCostOfCapitalDateOfAnnualImpairmentTest
Namespace Prefix: | rock_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_GoodwillLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration