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Equity-Based Compensation (Summary of Compensation Expense Connection with Awards) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation [Abstract] |  |  |
Expense recognized under terminated plans | $ 0 rock_StockCompensationExpenseRecognizedUnderTerminatedPlans | $ 0 rock_StockCompensationExpenseRecognizedUnderTerminatedPlans | $ 18 rock_StockCompensationExpenseRecognizedUnderTerminatedPlans
Expense recognized under the Plan | 3,150 rock_StockCompensationExpenseRecognizedUnderEquityIncentivePlan | 2,564 rock_StockCompensationExpenseRecognizedUnderEquityIncentivePlan | 3,130 rock_StockCompensationExpenseRecognizedUnderEquityIncentivePlan
Total stock compensation expense | 3,150 us-gaap_ShareBasedCompensation | 2,564 us-gaap_ShareBasedCompensation | 3,148 us-gaap_ShareBasedCompensation
Tax benefits recognized related to stock compensation expense | $ 1,229 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | $ 1,000 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | $ 1,228 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense

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- Definition Stock Compensation Expense Recognized Under Equity Incentive Plan + References No definition available. + Details
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- Definition Stock compensation expense recognized under terminated plans. + References No definition available. + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method. + 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 + Details
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- Details
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