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Consolidated Statements of Shareholders' Equity (Parenthetical) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Statement of Stockholders' Equity [Abstract] |  |  |
Adjustment to retirement benefit liability, taxes | $ 9 rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentTax1 | $ 30 rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentTax1 | $ 40 rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentTax1
Adjustment to post-retirement healthcare benefit liability, taxes | 830 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax | 28 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax | 290 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Unrealized loss on cash flow hedge | $ 82 us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax |  |

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- Definition Amount of tax (expense) benefit, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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- Definition Amount of tax expense (benefit), before reclassification adjustments, related to increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580 + Details
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