Goodwill and Related Intangible Assets (Schedule of Acquired Intangible Assets) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Finite-lived intangible assets	84,493	$ 84,524
Total acquired intangible assets, Gross Carrying Amount	126,278rock_AcquiredIntangibleAssetsTotal	127,244rock_AcquiredIntangibleAssetsTotal
Accumulated Amortization, Finite-lived intangible assets	45,839	45,029
Total acquired intangible assets, Accumulated Amortization	45,839	45,029
Trademarks
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Finite-lived intangible assets	4,493	3,886
Accumulated Amortization, Finite-lived intangible assets	1,949	1,827
Unpatented technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Finite-lived intangible assets	24,527	24,527
Accumulated Amortization, Finite-lived intangible assets	9,210	8,768
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Finite-lived intangible assets	52,336	52,974
Accumulated Amortization, Finite-lived intangible assets	31,775	31,554
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Finite-lived intangible assets	1,807	1,807
Accumulated Amortization, Finite-lived intangible assets	1,575	1,550
Backlog
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Finite-lived intangible assets	1,330	1,330
Accumulated Amortization, Finite-lived intangible assets	1,330	1,330
Trademarks
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount, Indefinite-lived intangible assets	41,785	42,720
Accumulated Amortization, Indefinite-lived intangible assets	0	$ 0
Minimum \| Trademarks
Finite-Lived Intangible Assets [Line Items]
Estimated Life	2 years
Minimum \| Unpatented technology
Finite-Lived Intangible Assets [Line Items]
Estimated Life	5 years
Minimum \| Customer relationships
Finite-Lived Intangible Assets [Line Items]
Estimated Life	5 years
Minimum \| Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Estimated Life	4 years
Minimum \| Backlog
Finite-Lived Intangible Assets [Line Items]
Estimated Life	1 year
Maximum \| Trademarks
Finite-Lived Intangible Assets [Line Items]
Estimated Life	15 years
Maximum \| Unpatented technology
Finite-Lived Intangible Assets [Line Items]
Estimated Life	20 years
Maximum \| Customer relationships
Finite-Lived Intangible Assets [Line Items]
Estimated Life	16 years
Maximum \| Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Estimated Life	10 years
Maximum \| Backlog
Finite-Lived Intangible Assets [Line Items]
Estimated Life	2 years

Acquired Intangible Assets, Total.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

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-Topic 350

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