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Income Taxes - Narrative (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating Loss Carryforwards [Line Items]      
Federal statutory tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Alternative minimum tax credits, non-expiring $ 1,000,000us-gaap_TaxCreditCarryforwardAmount    
Excess tax benefit from stock option exercise 1,800,000us-gaap_UnrecognizedTaxBenefits    
Additional tax expense related to penalties and interest incurred     99,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense
State      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforward 2,800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Internal Revenue Service (IRS)      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforward 1,000,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
Cash benefits received from operating loss carrybacks   2,700,000orn_RefundsReceivedFromApplicationOfOperatingLossCarrybacks
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
$ 13,100,000orn_RefundsReceivedFromApplicationOfOperatingLossCarrybacks
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember