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Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2018
Market Financial Group, Ltd and Austin Consulting Group Inc. (MFG) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jan. 01, 2018
McGregor & Associates (M&A) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Mar. 01, 2018
Five Other Acquisitions [Member]
Business Acquisition [Line Items]
Effective Year of Acquisition	2018

X

- Definition

Business acquisition effective date of acquisition year.

+ References

No definition available.

+ Details

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- Definition

Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e845-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e848-128460

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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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.

+ Details

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- Details

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X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_McGregorAndAssociatesMAndAMember
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X

- Details

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