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Marketable Securities (Schedule of classified marketable securities) (Details) (USD $)
 9 Months Ended 12 Months Ended
Apr. 30, 2015
Jul. 31, 2014
Available-for-sale - Fair Value $ 1,477,072us-gaap_AvailableForSaleSecurities $ 1,354,213us-gaap_AvailableForSaleSecurities
Noncurrent [Member]    
Available-for-sale - Cost 1,156,952us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 1,117,801us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Available-for-sale - Gross Unrealized Gains 320,120us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 236,412us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Available-for-sale - Gross Unrealized Losses      
Available-for-sale - Fair Value 1,477,072us-gaap_AvailableForSaleSecurities
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 1,354,213us-gaap_AvailableForSaleSecurities
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Noncurrent [Member] | Mutual Funds [Member]    
Available-for-sale - Cost 716,050us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mays_MutualFundsMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 691,047us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mays_MutualFundsMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Available-for-sale - Gross Unrealized Gains 141,481us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mays_MutualFundsMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 129,173us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mays_MutualFundsMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Available-for-sale - Gross Unrealized Losses      
Available-for-sale - Fair Value 857,531us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mays_MutualFundsMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 820,220us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mays_MutualFundsMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Noncurrent [Member] | Equity Securities [Member]    
Available-for-sale - Cost 440,902us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 426,754us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Available-for-sale - Gross Unrealized Gains 178,639us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 107,239us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember
Available-for-sale - Gross Unrealized Losses      
Available-for-sale - Fair Value $ 619,541us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember		 $ 533,993us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
/ mays_MarketableSecuritiesAxis
= mays_NoncurrentMember