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Goodwill and Intangible Assets - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]      
Intangible assets, net $ 102,122,000us-gaap_IntangibleAssetsNetExcludingGoodwill   $ 108,593,000us-gaap_IntangibleAssetsNetExcludingGoodwill
Amortization expense for intangible assets 2,400,000us-gaap_AmortizationOfIntangibleAssets 1,100,000us-gaap_AmortizationOfIntangibleAssets  
Estimated amortization expense for the remainder of 2015 7,600,000us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear    
Estimated amortization expense for 2016 10,000,000us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo    
Estimated amortization expense for 2017 9,900,000us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree    
Estimated amortization expense for 2018 9,200,000us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour    
Estimated amortization expense for 2019 9,100,000us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive    
Estimated amortization expense for 2020 9,100,000omi_FiniteLivedIntangibleAssetsAmortizationExpenseSixthYear    
Domestic Segment      
Finite-Lived Intangible Assets [Line Items]      
Intangible assets, net 64,600,000us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_StatementBusinessSegmentsAxis
= omi_DomesticMember
   
International Segment      
Finite-Lived Intangible Assets [Line Items]      
Intangible assets, net $ 37,500,000us-gaap_IntangibleAssetsNetExcludingGoodwill
/ us-gaap_StatementBusinessSegmentsAxis
= omi_InternationalMember