Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
Jul. 31, 2016	Jul. 31, 2015	Jul. 31, 2014
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of participants contribution	20.00%
Percentage of discretionary matching contributions	50.00%
Defined benefit plan compensation	6.00%
Company's cost for contributions to the plan	$ 1.4	$ 1.3	$ 1.1
Employment period contributions, description	First five years
401(k) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Class B common stock to the Plan for matching contributions	94,712	70,843	72,281

Employment period contributions description.

No definition available.

Name:	idt_DefinedContributionPlanEmployerDiscretionaryContributionVestingPeriod
Namespace Prefix:	idt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Contribution Plan Employer Matching Contribution In Employer Shares.

+ References

No definition available.

+ Details

Name:	idt_DefinedContributionPlanEmployerMatchingContributionInEmployerShares
Namespace Prefix:	idt_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan employer matching contribution limit percent.

+ References

No definition available.

+ Details

Name:	idt_DefinedContributionPlanEmployerMatchingContributionLimitPercent
Namespace Prefix:	idt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DefinedContributionPlanNameAxis=idt_DefinedContributionPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: