Goodwill and Identifiable Intangible Assets - Goodwill (Details) $ in Millions	9 Months Ended
Sep. 30, 2017 USD ($)
Goodwill [Roll Forward]
Balance as of December 31, 2016	$ 835.8
Additions	46.4
Foreign exchange and other adjustments	19.0
Balance as of September 30, 2017	901.1
Aviation segment
Goodwill [Roll Forward]
Balance as of December 31, 2016	266.8
Additions	46.4
Foreign exchange and other adjustments	8.9
Balance as of September 30, 2017	322.2
Land segment
Goodwill [Roll Forward]
Balance as of December 31, 2016	496.7
Additions	0.0
Foreign exchange and other adjustments	9.9
Balance as of September 30, 2017	506.6
Marine segment
Goodwill [Roll Forward]
Balance as of December 31, 2016	72.3
Additions	0.0
Foreign exchange and other adjustments	0.1
Balance as of September 30, 2017	$ 72.4

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

+ Details

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

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

+ Details

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

- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

+ Details

Name:	us-gaap_GoodwillForeignCurrencyTranslationGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=int_AviationSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=int_LandSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=int_MarineSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: