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Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2019
Inverson Holding Company - Company, LLC (IHC) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jan. 01, 2019
James Brown Inc. (JBI) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jan. 01, 2019
Nine Other Acquisition [Member]
Business Acquisition [Line Items]
Effective Year of Acquisition	2019

X

- Definition

Business acquisition effective date of acquisition year.

+ References

No definition available.

+ Details

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Namespace Prefix:	ajg_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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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X

- 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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Period Type:	duration

X

- Details

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Namespace Prefix:
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Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_JamesBrownIncMember
Namespace Prefix:
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Period Type:

X

- Details

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