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Employee Retirement Plans (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 employee | Dec. 31, 2013 agreement | Dec. 31, 2012 | Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Projected benefit obligation | $ 1,961 us-gaap_DefinedBenefitPlanBenefitObligation | $ 2,179 us-gaap_DefinedBenefitPlanBenefitObligation | $ 2,478 us-gaap_DefinedBenefitPlanBenefitObligation | $ 2,653 us-gaap_DefinedBenefitPlanBenefitObligation
Net liability | 2,116 us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet | 2,328 us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet | 2,555 us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet |
Number of employees | 2,416 rock_NumberOfEmployees |  |  |
Employees represented by unions through CBAs, percentage | 12.00%us-gaap_MultiemployerPlansCollectiveBargainingArrangementPercentageOfParticipants |  |  |
Number of CBAs expired and successfully renegotiated |  | 3 rock_NumberOfCollectiveBargainingAgreements |  |
Company's maximum contribution percentage | 5.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent | 5.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent | 5.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent |
Sheet Metal Workers Pension Plan Of Northern California [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
CBAs expiration date | Jun. 05, 2015 |  |  |
Sheet Metal Workers National Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
CBAs expiration date | Apr. 30, 2015 |  |  |
National Integrated Group Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
CBAs expiration date | Apr. 15, 2015 |  |  |
Foreign Pension Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Projected benefit obligation | 410 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPensionPlansDefinedBenefitMember | 441 us-gaap_DefinedBenefitPlanBenefitObligation / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPensionPlansDefinedBenefitMember |  |
Pre-tax accumulated other comprehensive loss - retirement benefit liability adjustment | 113 us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeMinimumPensionLiabilityBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPensionPlansDefinedBenefitMember | 73 us-gaap_DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeMinimumPensionLiabilityBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPensionPlansDefinedBenefitMember |  |
Net liability | $ 297 us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPensionPlansDefinedBenefitMember | $ 368 us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_ForeignPensionPlansDefinedBenefitMember |  |

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- Definition Number of collective bargaining agreements. + References No definition available. + Details
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- Definition Number Of Employees + References No definition available. + Details
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- Definition Before tax amount of additional minimum pension liability not yet recognized as a net periodic pension cost. + 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 (i) -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 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The aggregate net amount recognized in the balance sheet associated with the defined benefit plan(s). Will normally be the same as the Defined Benefit Plan, Funded Status of Plan, Total. + 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 (c) -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 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 + Details
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- Definition 1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. + 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 (a) -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 Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
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- Definition Expiration date of the collective bargaining arrangement requiring contributions to the multiemployer plan, in CCYY-MM-DD format. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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- Definition Percentage of employee participants subject to a collective bargaining arrangement to all participants in a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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