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Investments: (Details) - USD ($)
12 Months Ended
Dec. 30, 2017
Dec. 31, 2016
Dec. 26, 2015
Summary of marketable securities classified as available-for-sale securities      
Equity securities, Cost   $ 215,800  
Equity securities, Fair Value   199,900  
Unrealized gains and losses for marketable securities classified as available-for-sale securities in accumulated other comprehensive loss      
Unrealized losses   (15,900) $ (52,800)
Net unrealized losses   (15,900) (52,800)
Realized gains and losses recognized on sales of available-for-sale marketable securities      
Realized gains $ 10,300   13,400
Realized losses (8,900) (12,600) (36,200)
Net realized losses $ 1,400 $ (12,600) $ (22,800)