Shareholders' Equity - Unrecognized Compensation Cost (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Equity [Abstract]
Total unrecognized compensatoin cost related to unvested share-based payment awards	$ 24.6
Period for recognition of unrecognized compensation cost related to unvested share-based payment awards	10 months 24 days
Year Ended December 31,
2020	$ 16.2
2021	7.2
2022	1.1
2023	0.0
2024	0.0
Total	$ 24.6

Unrecognized compensation cost expected to be recognized in year five of the five succeeding years.

No definition available.

Name:	int_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostToBeRecognizedInYearFive
Namespace Prefix:	int_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unrecognized compensation cost expected to be recognized in year four of the five succeeding years.

+ References

No definition available.

+ Details

Name:	int_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostToBeRecognizedInYearFour
Namespace Prefix:	int_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unrecognized compensation cost expected to be recognized in year one of the five succeeding years.

+ References

No definition available.

+ Details

Name:	int_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostToBeRecognizedInYearOne
Namespace Prefix:	int_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unrecognized compensation cost expected to be recognized in year three of the five succeeding years.

+ References

No definition available.

+ Details

Name:	int_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostToBeRecognizedInYearThree
Namespace Prefix:	int_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unrecognized compensation cost expected to be recognized in year two of the five succeeding years.

+ References

No definition available.

+ Details

Name:	int_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostToBeRecognizedInYearTwo
Namespace Prefix:	int_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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