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Share-Based Compensation (Components Of Share-Based Compensation Expense) (Details) (USD $)
In Thousands, unless otherwise specified
 4 Months Ended
Jan. 18, 2015
Jan. 19, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense $ 3,885us-gaap_ShareBasedCompensation $ 3,801us-gaap_ShareBasedCompensation
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense 999us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 1,289us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Performance-Vested Stock Awards [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense 1,081us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= jack_PerformanceVestedStockAwardsMember		 1,497us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= jack_PerformanceVestedStockAwardsMember
Nonvested Stock Awards [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense 61us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= jack_NonvestedStockAwardsMember		 173us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= jack_NonvestedStockAwardsMember
Nonvested Stock Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense $ 1,744us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= jack_NonvestedStockUnitsMember		 $ 842us-gaap_ShareBasedCompensation
/ us-gaap_AwardTypeAxis
= jack_NonvestedStockUnitsMember