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Pension and Other Post-Retirement Benefit Plans (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Aggregate defined contribution plan expenses $ 3.3us-gaap_DefinedContributionPlanCostRecognized $ 4.7us-gaap_DefinedContributionPlanCostRecognized $ 4.9us-gaap_DefinedContributionPlanCostRecognized
U.S. Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Settlements plan assets 29.6us-gaap_DefinedBenefitPlanSettlementsPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
0us-gaap_DefinedBenefitPlanSettlementsPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
Settlement charge (3.2)us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
0us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
0us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
Expected employer contribution in 2014 0.7us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
   
EPT Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of U.S. plan assets 93.00%omg_DefinedBenefitPlanPlanAssetsAsPercentageOfUnitedStatesPlans
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= omg_EaglepicherTechnologiesPensionPlansMember
   
Non-U.S. Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Settlements plan assets 0us-gaap_DefinedBenefitPlanSettlementsPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
0us-gaap_DefinedBenefitPlanSettlementsPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
 
Expected employer contribution in 2014 10.7us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_ForeignPensionPlansDefinedBenefitMember
   
Battery Technologies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Settlement charge $ 3.2us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments1
/ us-gaap_StatementBusinessSegmentsAxis
= omg_BatteryTechnologiesMember
   
EaglePicher Technologies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Number of benefit plans assumed in acquisition 4omg_BusinessAcquisitionNumberOfBenefitPlansAssumed
/ us-gaap_BusinessAcquisitionAxis
= omg_EaglepicherTechnologiesMember
   
Number of frozen defined benefit pension plans with liabilities company assumed in acquisition 2omg_BusinessAcquisitionNumberOfFrozenDefinedBenefitPensionPlansCompanyAssumedLiabilities
/ us-gaap_BusinessAcquisitionAxis
= omg_EaglepicherTechnologiesMember