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Long-Term Debt (Narrative) (Note Payable to SMBC) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended
Jan. 31, 2011
Dec. 31, 2014
Dec. 31, 2013
Debt Instrument [Line Items]      
Long-term Debt   $ 1,192.6us-gaap_LongTermDebt $ 942.2us-gaap_LongTermDebt
Senior Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Long-term Debt   350.0us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= onnn_SeniorRevolvingCreditFacilityMember		 120.0us-gaap_LongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= onnn_SeniorRevolvingCreditFacilityMember
SANYO Electric [Member]      
Debt Instrument [Line Items]      
Period of debt, in years 7 years    
Principal amount of debt 377.5us-gaap_DebtInstrumentFaceAmount
/ us-gaap_FinancialInstrumentAxis
= onnn_SanyoElectricMember		    
Long-term Debt   235.9us-gaap_LongTermDebt
/ us-gaap_FinancialInstrumentAxis
= onnn_SanyoElectricMember		 273.7us-gaap_LongTermDebt
/ us-gaap_FinancialInstrumentAxis
= onnn_SanyoElectricMember
Basis spread on variable rate 1.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_FinancialInstrumentAxis
= onnn_SanyoElectricMember		    
Principal payment of loan, quarterly 9.4onnn_DebtInstrumentMonthlyPaymentPrincipal
/ us-gaap_FinancialInstrumentAxis
= onnn_SanyoElectricMember		    
Remaining balance of loan due in January 2018 $ 122.7onnn_RemainingBalanceOfLoanDueInJanuaryTwoThousandEighteen
/ us-gaap_FinancialInstrumentAxis
= onnn_SanyoElectricMember