Debt Instrument Maturity Year

No definition available.

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

+ Details

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

- Definition

The fair value of notes issued in noncash investing and financing activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
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+ Details

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

- Details

Name:	us-gaap_ExtinguishmentOfDebtAxis=ajg_FixedRateOfFivePointSevenFivePercentBalloonDueMarchTwoTwoThousandFiftyThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Credit and Other Debt Agreements - Additional Information (Detail) - USD ($) $ in Millions	Mar. 02, 2023	Mar. 31, 2023	Dec. 31, 2022
Fixed Rate of 5.75%, Balloon Due March 2, 2053 [Member]
Debt Instrument [Line Items]
Periodic payment of interest		5.75%	5.75%
Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold	$ 950.0
Weighted average interest rate	5.05%
Realized a cash gain (loss) on hedging transaction	$ 112.7
Derivative instrument, term	10 years
Unsecured Senior Notes [Member] \| Fixed Rate of 5.50%, Balloon Due May 2, 2033 [Member]
Debt Instrument [Line Items]
Notes issued and sold	$ 350.0
Periodic payment of interest	5.50%
Periodic payments of interest and principal, expiry year	2033
Unsecured Senior Notes [Member] \| Fixed Rate of 5.75%, Balloon Due March 2, 2053 [Member]
Debt Instrument [Line Items]
Notes issued and sold	$ 600.0
Periodic payment of interest	5.75%
Periodic payments of interest and principal, expiry year	2053