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Contracts with Customers - Summary of Changes in Deferred Revenue Balances (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	$ 457.7
Incremental deferred revenue	348.5
Revenue recognized during the nine-month period ended September 30, 2019 included in deferred revenue at December 31, 2018	(336.6)
Deferred revenue recognized from business acquisitions	24.3
Deferred revenue ending balance	493.9
Brokerage [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	284.7
Incremental deferred revenue	239.2
Revenue recognized during the nine-month period ended September 30, 2019 included in deferred revenue at December 31, 2018	(221.0)
Deferred revenue recognized from business acquisitions	24.3
Deferred revenue ending balance	327.2
Risk Management [Member]
Deferred Revenue Arrangement [Line Items]
Deferred revenue beginning balance	173.0
Incremental deferred revenue	109.3
Revenue recognized during the nine-month period ended September 30, 2019 included in deferred revenue at December 31, 2018	(115.6)
Deferred revenue ending balance	$ 166.7

X

- Definition

Contract with customer, liability, additions.

+ References

No definition available.

+ Details

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Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130532-203044

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X

- Definition

Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration from customer has been received or is due, from business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130551-203045

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X

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

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- 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.

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