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Investments: (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 26, 2015
Dec. 27, 2014
Summary of marketable securities classified as available-for-sale securities      
Equity securities, Cost $ 215,800 $ 280,600  
Equity securities, Fair Value 199,900 227,800  
Unrealized gains and losses for marketable securities classified as available-for-sale securities in accumulated other comprehensive loss      
Unrealized gains     $ 3,300
Unrealized losses (15,900) (52,800) (63,100)
Net unrealized losses (15,900) (52,800) (59,800)
Realized gains and losses recognized on sales of available-for-sale marketable securities      
Realized gains   13,400 40,900
Realized losses (12,600) (36,200) (32,200)
Net realized losses $ (12,600) $ (22,800) $ 8,700