Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail) $ in Millions	Jun. 30, 2023 USD ($)
Senior Notes [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, Thereafter	$ 2,550.0
Contractual Obligations, Payments Due by Period, Total	2,550.0
Note Purchase Agreements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2024	425.0
Contractual Obligations, Payments Due by Period, 2025	200.0
Contractual Obligations, Payments Due by Period, 2026	640.0
Contractual Obligations, Payments Due by Period, 2027	478.0
Contractual Obligations, Payments Due by Period, Thereafter	2,205.0
Contractual Obligations, Payments Due by Period, Total	3,948.0
Credit Agreement [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	495.0
Contractual Obligations, Payments Due by Period, Total	495.0
Premium Financing Debt Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	191.2
Contractual Obligations, Payments Due by Period, Total	191.2
Interest On Debt [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	137.0
Contractual Obligations, Payments Due by Period, 2024	265.2
Contractual Obligations, Payments Due by Period, 2025	251.4
Contractual Obligations, Payments Due by Period, 2026	232.6
Contractual Obligations, Payments Due by Period, 2027	214.1
Contractual Obligations, Payments Due by Period, Thereafter	2,393.8
Contractual Obligations, Payments Due by Period, Total	3,494.1
Total Debt Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	823.2
Contractual Obligations, Payments Due by Period, 2024	690.2
Contractual Obligations, Payments Due by Period, 2025	451.4
Contractual Obligations, Payments Due by Period, 2026	872.6
Contractual Obligations, Payments Due by Period, 2027	692.1
Contractual Obligations, Payments Due by Period, Thereafter	7,148.8
Contractual Obligations, Payments Due by Period, Total	10,678.3
Operating Lease Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	50.0
Contractual Obligations, Payments Due by Period, 2024	101.7
Contractual Obligations, Payments Due by Period, 2025	84.2
Contractual Obligations, Payments Due by Period, 2026	68.6
Contractual Obligations, Payments Due by Period, 2027	54.4
Contractual Obligations, Payments Due by Period, Thereafter	100.2
Contractual Obligations, Payments Due by Period, Total	459.1
Less Sublease Arrangements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Receivable by Period, 2023	(0.8)
Contractual Obligations, Payments Receivable by Period, 2024	(1.2)
Contractual Obligations, Payments Receivable by Period, 2025	(0.7)
Contractual Obligations, Payments Receivable by Period, 2026	(0.6)
Contractual Obligations, Payments Receivable by Period, 2027	(0.5)
Contractual Obligations, Payments Receivable by Period, Total	(3.8)
Outstanding Purchase Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	83.6
Contractual Obligations, Payments Due by Period, 2024	94.2
Contractual Obligations, Payments Due by Period, 2025	51.1
Contractual Obligations, Payments Due by Period, 2026	19.3
Contractual Obligations, Payments Due by Period, 2027	15.7
Contractual Obligations, Payments Due by Period, Thereafter	49.5
Contractual Obligations, Payments Due by Period, Total	313.4
Total Contractual Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2023	956.0
Contractual Obligations, Payments Due by Period, 2024	884.9
Contractual Obligations, Payments Due by Period, 2025	586.0
Contractual Obligations, Payments Due by Period, 2026	959.9
Contractual Obligations, Payments Due by Period, 2027	761.7
Contractual Obligations, Payments Due by Period, Thereafter	7,298.5
Contractual Obligations, Payments Due by Period, Total	$ 11,447.0

Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]

No definition available.

Name:	ajg_CommitmentsContingenciesAndOffBalanceSheetArrangementsLineItems
Namespace Prefix:	ajg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Recorded unconditional purchase obligation due after fourth year.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationDueAfterFourthYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Recorded unconditional purchase obligation receivable.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationReceivable
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded unconditional purchase obligation receivable in fourth year.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationReceivableInFourthYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded unconditional purchase obligation receivable in remainder of fiscal year.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationReceivableInRemainderOfFiscalYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded unconditional purchase obligation receivable in second year.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationReceivableInSecondYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded unconditional purchase obligation receivable in third year.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationReceivableInThirdYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded unconditional purchase obligation receivable within one year.

+ References

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationReceivableWithinOneYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of the recorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 440
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482648/440-10-50-6

+ Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recorded unconditional purchase obligation to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationDueInFourthYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recorded unconditional purchase obligation to be paid in remainder of current fiscal year.

+ References

No definition available.

+ Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationDueInRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recorded unconditional purchase obligation to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationDueInSecondYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recorded unconditional purchase obligation to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationDueInThirdYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recorded unconditional purchase obligation to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationDueWithinOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_NotePurchaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_UnsecuredMulticurrencyCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_PremiumFinancingDebtFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_InterestOnDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_DebtObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_OperatingLeaseObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_SubleaseArrangementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_OutstandingPurchaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis=ajg_ContractualObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: