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Equity-Based Compensation (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2014 | Jan. 31, 2014 | Jan. 31, 2013 | Jan. 31, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Typical vesting period, in years |  |  |  |  | 4 years |  |
Number of shares available for issuance under the Plan |  |  |  |  | 376,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant |  |
Options granted |  |  |  |  | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Number of restricted stock units credited to participant accounts |  |  |  |  | 647,371 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount | 614,888 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount |
Market price of common stock, per share |  |  |  |  | 16.26 rock_MarketPriceOfCommonStockPerShare |  |
Requisite vesting age |  |  |  |  | 60 years |  |
Intrinsic value of options exercisable |  |  |  |  | 1,393 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 |  |
Weighted average remaining life of options exercisable |  |  |  |  | 4 years |  |
Unrecognized compensation cost |  |  |  |  | 4,300 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |
Weighted average cost recognition period, in years |  |  |  |  | 3 years 3 months 18 days |  |
2005 Equity Incentive Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Maximum number of shares that may be issued under the Plan |  |  |  |  | 3,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_StatementEquityComponentsAxis = rock_TwoThousandFiveEquityIncentivePlanMember |  |
Maximum number of shares that may be issued in connection with grants of incentive stock options and rights |  |  |  |  | 900,000 rock_MaximumNumberOfSharesIssuedInConnectionWithGrantsOfIncentiveStockOptionsAndRights / us-gaap_StatementEquityComponentsAxis = rock_TwoThousandFiveEquityIncentivePlanMember |  |
Performance Stock Units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Percentage of target performance stock units earned |  |  |  |  |  | 50.00%rock_EquityBasedCompensationPercentOfTargetedPerformanceStockUnitsEarned / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 58.30%rock_EquityBasedCompensationPercentOfTargetedPerformanceStockUnitsEarned / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember
Targeted Performance Stock Units |  |  |  |  |  | 237,000 rock_TargetedPerformanceStockUnits / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 280,000 rock_TargetedPerformanceStockUnits / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember
Performance stock units outstanding |  |  |  |  |  | 237,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 280,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember
Shares awarded | 19,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 212,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 304,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 295,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember |  |  |
Performance stock unit grant-date fair value | $ 319 rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | $ 3,914 rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | $ 4,123 rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | $ 4,152 rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember |  |  |
Performance stock units earned |  |  |  |  |  | 114,000 rock_PerformanceStockUnitsEarned / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember | 163,200 rock_PerformanceStockUnitsEarned / us-gaap_StatementEquityComponentsAxis = rock_PerformanceStockUnitsMember
Service period |  |  |  |  | 5 years |  |
Closing price period for determination of cash conversion |  |  |  |  | 90 days |  |
Management Stock Purchase Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Performance stock units outstanding |  |  |  |  | 224,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember |  |
Closing price period for determination of cash conversion |  |  |  |  | 200 days |  |
Weighted average fair value of per unit awarded |  |  |  |  | 15.68 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember | 15.97 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember |
Number of restricted stock units credited to participant accounts |  |  |  |  | 119,105 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember | 132,037 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember | 253,587 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember
Accrues interest |  |  |  |  | 2.00%rock_PercentageOfAccruesInterest / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember |  |
Lump sum payment option one |  |  |  |  | 1 rock_SharebasedCompensationArrangementbySharebasedPaymentAwardLumpSumPaymentOptionOne / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember |  |
Lump sum payment option two |  |  |  |  | 5 rock_SharebasedCompensationArrangementbySharebasedPaymentAwardLumpSumPaymentOptionTwo / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember |  |
Lump sum payment option three |  |  |  |  | 10 rock_SharebasedCompensationArrangementbySharebasedPaymentAwardLumpSumPaymentOptionThree / us-gaap_StatementEquityComponentsAxis = rock_ManagementStockPurchasePlanMember |  |
Maximum | Performance Stock Units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Performance stock unit vesting period |  |  |  |  | 3 years |  |
Minimum | Performance Stock Units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Performance stock unit vesting period |  |  |  |  | 1 year |  |
Unvested Restricted Stock Units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of restricted stock units credited to participant accounts |  |  |  |  | 62,455 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount / us-gaap_AwardTypeAxis = rock_UnvestedRestrictedStockUnitMember | 47,268 rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount / us-gaap_AwardTypeAxis = rock_UnvestedRestrictedStockUnitMember |

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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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