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Share-Based Compensation (Schedule Of Share-Based Awards Granted) (Details)
 4 Months Ended
Jan. 18, 2015
Stock Options [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares 123,042us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Performance-Vested Stock Awards [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares 40,594us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= jack_PerformanceVestedStockAwardsMember
Nonvested Stock Units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares 87,081us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= jack_NonvestedStockUnitsMember