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Fair Value Of Financial Instruments (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Feb. 28, 2015
Aug. 31, 2014
Fair Value Disclosure [Line Items]    
Cash equivalents carried at cost $ 34.2us-gaap_CashEquivalentsAtCarryingValue $ 34.4us-gaap_CashEquivalentsAtCarryingValue
2013 and 2011 Fixed Rate Notes [Member]    
Fair Value Disclosure [Line Items]    
Long-term Debt, Fair Value 432.9us-gaap_LongTermDebtFairValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandThirteenAndTwoThousandElevenFixedRateNotesMember		 437.8us-gaap_LongTermDebtFairValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandThirteenAndTwoThousandElevenFixedRateNotesMember
Fixed rate notes, carrying value, including accrued interest 432.9sonc_LongTermDebtFixedRateNotesIncludingAccruedInterestCarryingValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandThirteenAndTwoThousandElevenFixedRateNotesMember		 437.8sonc_LongTermDebtFixedRateNotesIncludingAccruedInterestCarryingValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandThirteenAndTwoThousandElevenFixedRateNotesMember
2011 Variable Rate Notes [Member]    
Fair Value Disclosure [Line Items]    
Long-term Debt, Fair Value 48.6us-gaap_LongTermDebtFairValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandElevenVariableRateNotesMember		 0us-gaap_LongTermDebtFairValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandElevenVariableRateNotesMember
Fixed rate notes, carrying value, including accrued interest $ 48.6sonc_LongTermDebtFixedRateNotesIncludingAccruedInterestCarryingValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandElevenVariableRateNotesMember		 $ 0sonc_LongTermDebtFixedRateNotesIncludingAccruedInterestCarryingValue
/ us-gaap_DebtInstrumentAxis
= sonc_TwoThousandElevenVariableRateNotesMember