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v2.4.1.9

Other Postretirement Benefits (Components of Net Periodic Postretirement Benefit Cost) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Service cost | $ 0 us-gaap_DefinedBenefitPlanServiceCost | $ 0 us-gaap_DefinedBenefitPlanServiceCost | $ 0 us-gaap_DefinedBenefitPlanServiceCost
Interest cost | 88 us-gaap_DefinedBenefitPlanInterestCost | 80 us-gaap_DefinedBenefitPlanInterestCost | 110 us-gaap_DefinedBenefitPlanInterestCost
Amortization of unrecognized prior service cost | 15 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit | 14 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit | 14 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
Net periodic pension cost | 103 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost | 94 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost | 124 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
Discount rate | 3.74%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate | 4.45%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate | 3.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Ultimate healthcare trend rates | 4.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate |  |
Other Postretirement Benefits |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Service cost | 16 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 16 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 15 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Interest cost | 255 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 203 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 224 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Amortization of unrecognized prior service cost | 44 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 0 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | (1)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Loss amortization (2) | 78 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 123 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 111 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Net periodic pension cost | $ 393 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | $ 342 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | $ 349 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Discount rate | 3.70%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 4.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 3.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Annual rate of increase in the per capita cost of: Medical costs before age 65 | 8.00%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearMedicalCostsBeforeAge65 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 8.50%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearMedicalCostsBeforeAge65 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 9.00%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearMedicalCostsBeforeAge65 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Annual rate of increase in the per capita cost of: Medical costs after age 65 | 7.00%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearMedicalCostsAfterAge65 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 7.30%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearMedicalCostsAfterAge65 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 7.50%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearMedicalCostsAfterAge65 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember
Annual rate of increase in the per capita cost of: Prescription drug costs | 9.00%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearPrescriptionDrugCosts / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 7.80%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearPrescriptionDrugCosts / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember | 8.00%rock_DefinedBenefitPlanHealthCareCostTrendRateAssumedforNextFiscalYearPrescriptionDrugCosts / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_DefinedBenefitPostretirementHealthCoverageMember

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- Definition Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Medical Costs After Age 65 + References No definition available. + Details
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- Definition Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Medical Costs Before Age 65 + References No definition available. + Details
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- Definition Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year, Prescription Drug Costs + References No definition available. + Details
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- Definition The amount of gains or losses recognized in net periodic benefit cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(4) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The ultimate trend rate for health care costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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