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Fair Value Measurements (Details Textual) (USD $)
In Millions, unless otherwise specified
Apr. 30, 2015
Jul. 31, 2014
Fair Value Assets Liabilities Quantitative Information [Line Items]    
Fair value of investments in hedge funds $ 9.1idt_InvestmentInHedgeFunds $ 9.5idt_InvestmentInHedgeFunds
Carrying value of investments 3.6us-gaap_FairValueEstimateNotPracticableCostMethodInvestments 1.8us-gaap_FairValueEstimateNotPracticableCostMethodInvestments
Long-term Investments [Member]    
Fair Value Assets Liabilities Quantitative Information [Line Items]    
Fair value of investments in hedge funds 9.1idt_InvestmentInHedgeFunds
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_OtherLongTermInvestmentsMember		 9.4idt_InvestmentInHedgeFunds
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_OtherLongTermInvestmentsMember
Other Current Assets [Member]    
Fair Value Assets Liabilities Quantitative Information [Line Items]    
Fair value of investments in hedge funds $ 0.1idt_InvestmentInHedgeFunds
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_OtherCurrentAssetsMember		 $ 0.1idt_InvestmentInHedgeFunds
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_OtherCurrentAssetsMember