Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail) $ in Millions	Dec. 31, 2019 USD ($)
Note Purchase Agreements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	$ 100.0
Contractual Obligations, Payments Due by Period, 2021	75.0
Contractual Obligations, Payments Due by Period, 2022	200.0
Contractual Obligations, Payments Due by Period, 2023	300.0
Contractual Obligations, Payments Due by Period, 2024	475.0
Contractual Obligations, Payments Due by Period, Thereafter	2,773.0
Contractual Obligations, Payments Due by Period, Total	3,923.0
Multi Currency Credit Agreement [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	520.0
Contractual Obligations, Payments Due by Period, Total	520.0
Premium Financing Debt Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	170.6
Contractual Obligations, Payments Due by Period, Total	170.6
Interest On Debt [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	173.6
Contractual Obligations, Payments Due by Period, 2021	167.5
Contractual Obligations, Payments Due by Period, 2022	161.8
Contractual Obligations, Payments Due by Period, 2023	152.5
Contractual Obligations, Payments Due by Period, 2024	134.7
Contractual Obligations, Payments Due by Period, Thereafter	574.7
Contractual Obligations, Payments Due by Period, Total	1,364.8
Total Debt Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	964.2
Contractual Obligations, Payments Due by Period, 2021	242.5
Contractual Obligations, Payments Due by Period, 2022	361.8
Contractual Obligations, Payments Due by Period, 2023	452.5
Contractual Obligations, Payments Due by Period, 2024	609.7
Contractual Obligations, Payments Due by Period, Thereafter	3,347.7
Contractual Obligations, Payments Due by Period, Total	5,978.4
Operating Lease Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	105.6
Contractual Obligations, Payments Due by Period, 2021	100.4
Contractual Obligations, Payments Due by Period, 2022	80.3
Contractual Obligations, Payments Due by Period, 2023	63.8
Contractual Obligations, Payments Due by Period, 2024	45.1
Contractual Obligations, Payments Due by Period, Thereafter	84.1
Contractual Obligations, Payments Due by Period, Total	479.3
Less Sublease Arrangements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	(0.6)
Contractual Obligations, Payments Due by Period, 2021	(0.6)
Contractual Obligations, Payments Due by Period, 2022	(0.3)
Contractual Obligations, Payments Due by Period, 2023	(0.3)
Contractual Obligations, Payments Due by Period, 2024	(0.2)
Contractual Obligations, Payments Due by Period, Thereafter	(0.7)
Contractual Obligations, Payments Due by Period, Total	(2.7)
Outstanding Purchase Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	49.9
Contractual Obligations, Payments Due by Period, 2021	38.2
Contractual Obligations, Payments Due by Period, 2022	23.2
Contractual Obligations, Payments Due by Period, 2023	9.0
Contractual Obligations, Payments Due by Period, 2024	5.7
Contractual Obligations, Payments Due by Period, Thereafter	17.4
Contractual Obligations, Payments Due by Period, Total	143.4
Total Contractual Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2020	1,119.1
Contractual Obligations, Payments Due by Period, 2021	380.5
Contractual Obligations, Payments Due by Period, 2022	465.0
Contractual Obligations, Payments Due by Period, 2023	525.0
Contractual Obligations, Payments Due by Period, 2024	660.3
Contractual Obligations, Payments Due by Period, Thereafter	3,448.5
Contractual Obligations, Payments Due by Period, Total	$ 6,598.4

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

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25383-109308

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- 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_LessSubleaseArrangementsMember
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: