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Share Repurchase Program (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
3 Months Ended 1 Months Ended
Nov. 30, 2014
Oct. 31, 2014
Aug. 31, 2014
Share Repurchase Program [Line Items]      
Shares repurchase authorized amount     $ 105.0us-gaap_StockRepurchaseProgramAuthorizedAmount1
Shares acquired through stock repurchase program 0.8us-gaap_TreasuryStockSharesAcquired    
Purchase of treasury stock 20.2us-gaap_TreasuryStockValueAcquiredCostMethod    
Remaining amount authorized for repurchase through share repurchase program 84.8us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1    
Weighted-average price per share $ 24.82us-gaap_TreasuryStockAcquiredAverageCostPerShare    
Accelerated Share Repurchase Program [Member]      
Share Repurchase Program [Line Items]      
Shares repurchase authorized amount   15.0us-gaap_StockRepurchaseProgramAuthorizedAmount1
/ us-gaap_ShareRepurchaseProgramAxis
= sonc_AcceleratedShareRepurchaseProgramMember
 
Shares acquired through stock repurchase program   0.6us-gaap_TreasuryStockSharesAcquired
/ us-gaap_ShareRepurchaseProgramAxis
= sonc_AcceleratedShareRepurchaseProgramMember
 
Purchase of treasury stock   $ 15.0us-gaap_TreasuryStockValueAcquiredCostMethod
/ us-gaap_ShareRepurchaseProgramAxis
= sonc_AcceleratedShareRepurchaseProgramMember