Income Taxes - Tax Effects On Deferred Tax Assets And Deferred Tax Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Employee benefit plans	$ 36,127	$ 28,892
Accrued liabilities not currently deductible	9,937	13,591
Finance charges	6,951	6,316
Capital leases	9,081	4,419
Net operating loss carryforwards	4,100	3,922
Other	14,150	8,643
Other	3,432us-gaap_DeferredTaxAssetsOther	2,083us-gaap_DeferredTaxAssetsOther
Total deferred tax assets	83,778us-gaap_DeferredTaxAssetsGross	67,866us-gaap_DeferredTaxAssetsGross
Less: valuation allowances	(9,639)	(5,250)
Net deferred tax assets	74,139us-gaap_DeferredTaxAssetsNet	62,616us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities:
Merchandise inventories	66,864	69,994
Goodwill	35,495	33,455
Property and equipment	21,578	20,938
Computer software	17,399	11,400
Insurance	539omi_DeferredTaxLiabilitiesInsurance	1,070omi_DeferredTaxLiabilitiesInsurance
Intangible assets	24,750	2,352
Other	1,410us-gaap_DeferredTaxLiabilitiesOther	585us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities	168,035us-gaap_DeferredTaxLiabilities	139,794us-gaap_DeferredTaxLiabilities
Net deferred tax liability	$ (93,896)us-gaap_DeferredTaxAssetsLiabilitiesNet	$ (77,178)us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred Tax Liabilities, Insurance

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the allowance for doubtful accounts.

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