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Investments: (Details) (USD $)
3 Months Ended
Mar. 28, 2015
Mar. 29, 2014
Dec. 27, 2014
Summary of marketable securities classified as available-for-sale securities      
Equity securities, Cost $ 238,600us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis   $ 526,600us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
Equity securities, Fair Value 225,200us-gaap_AvailableForSaleSecuritiesEquitySecuritiesCurrent   466,800us-gaap_AvailableForSaleSecuritiesEquitySecuritiesCurrent
Unrealized gains and losses for marketable securities classified as available-for-sale securities in accumulated other comprehensive loss      
Unrealized gains 3,900us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax   3,300us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Unrealized losses (17,300)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax   (63,100)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Net unrealized losses (13,400)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax   (59,800)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
Realized gains and losses recognized on sales of available-for-sale marketable securities      
Realized gains 13,400us-gaap_AvailableForSaleSecuritiesGrossRealizedGains 22,200us-gaap_AvailableForSaleSecuritiesGrossRealizedGains  
Realized losses (36,200)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses (12,800)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses  
Net realized gains (losses) $ (22,800)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet $ 9,400us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet