XML 80 R66.htm IDEA: XBRL DOCUMENT v3.10.0.1

Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail)	12 Months Ended
	Dec. 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
Pronto Insurance (PI) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jun. 05, 2018
Reassurance Holdings, Inc (RHI) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jul. 01, 2018
Buckman Mitchell, Inc. (BMI) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Nov. 01, 2018
Accompass Inc. (AI) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Dec. 01, 2018
42 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

Name:	ajg_BusinessAcquisitionEffectiveDateOfAcquisitionYear
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

+ Details

Name:	us-gaap_BusinessAcquisitionEffectiveDateOfAcquisition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

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

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

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_MarketFinancialGroupLtdAndAustinConsultingGroupIncMFGMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_McGregorAndAssociatesMAndAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_ProntoInsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_ReassuranceHoldingsIncRhiMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_BuckmanMitchellIncBmiMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_AccompassIncAiMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ajg_OtherAcquisitionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: