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

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- Definition

Stock Compensation Expense Recognized Under Equity Incentive Plan

+ References

No definition available.

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- Definition

Stock compensation expense recognized under terminated plans.

+ References

No definition available.

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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

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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

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