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Investments: (Details) - USD ($)
9 Months Ended
Sep. 24, 2016
Sep. 26, 2015
Dec. 26, 2015
Summary of marketable securities classified as available-for-sale securities      
Equity securities, Cost $ 215,800   $ 280,600
Equity securities, Fair Value 204,100   227,800
Unrealized gains and losses for marketable securities classified as available-for-sale securities in accumulated other comprehensive loss      
Unrealized losses (11,700)   (52,800)
Net unrealized losses (11,700)   $ (52,800)
Realized gains and losses recognized on sales of available-for-sale marketable securities      
Realized gains   $ 13,400  
Realized losses (12,600) (36,200)  
Net realized losses $ (12,600) $ (22,800)