Fair Value Measurements (Intangible Assets) (Details)	12 Months Ended
Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate (WACC)	15.00%
Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Risk Premium	1.00%
Royalty rate	2.00%
Future attributable revenues	77.00%
Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Risk Premium	1.00%
Royalty rate	2.00%
Future attributable revenues	97.00%

Rate of future attributable revenues, used as an input to measure fair value.

No definition available.

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

- Definition

Rate of return in excess of the risk-free rate of return that an investment is expected to yield, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Rate of payment to an owner for the use of property, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

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