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Commitments and Contingencies (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
U.S. defined contribution plan
Deferred Compensation Plans
Employer match for each 1% of the participants contributions up to 6% of the participants contributions (as a percent)	50.00%int_DefinedContributionPlanEmployerMatch / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = int_UnitedStatesDefinedContributionPlansMember
Percentage of eligible compensation up to 6% of the eligible compensation, matched 50% by employer	1.00%int_DefinedContributionPlanEmployerMatchEmployeeContributionLevelOne / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = int_UnitedStatesDefinedContributionPlansMember
Employer contribution limit per calendar year (as a percent of compensation)	6.00%int_DefinedContributionPlanEmployerContributionLimitPercentageOfCompensation / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = int_UnitedStatesDefinedContributionPlansMember
NQDC plan
Deferred Compensation Plans
Deferred compensation payable	2.3us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = int_NonqualifiedDeferredCompensationPlanMember	$ 0.9us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = int_NonqualifiedDeferredCompensationPlanMember

X

- Definition

The limit of annual employer contributions to the plan per calendar year as a percentage of compensation.

+ References

No definition available.

+ Details

Name:	int_DefinedContributionPlanEmployerContributionLimitPercentageOfCompensation
Namespace Prefix:	int_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the employer matching contributions for the employees' contributions.

+ References

No definition available.

+ Details

Name:	int_DefinedContributionPlanEmployerMatch
Namespace Prefix:	int_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the first level of employee contributions (percentage of compensation) which are matched by the employer.

+ References

No definition available.

+ Details

Name:	int_DefinedContributionPlanEmployerMatchEmployeeContributionLevelOne
Namespace Prefix:	int_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	int_DefinedContributionPlansLineItems
Namespace Prefix:	int_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer).

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationLiabilityClassifiedNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=int_NonqualifiedDeferredCompensationPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=int_UnitedStatesDefinedContributionPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: