Commitments, Contingencies and Off-Balance Sheet Arrangements - Contractual Obligations (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Note Purchase Agreements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	$ 100.0
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, Thereafter	2,423.0
Contractual Obligations, Payments Due by Period, Total	3,198.0
Multi Currency Credit Agreement [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	265.0
Contractual Obligations, Payments Due by Period, Total	265.0
Premium Financing Debt Facility [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	154.0
Contractual Obligations, Payments Due by Period, Total	154.0
Interest On Debt [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	138.0
Contractual Obligations, Payments Due by Period, 2020	132.5
Contractual Obligations, Payments Due by Period, 2021	127.9
Contractual Obligations, Payments Due by Period, 2022	122.5
Contractual Obligations, Payments Due by Period, 2023	113.1
Contractual Obligations, Payments Due by Period, Thereafter	444.5
Contractual Obligations, Payments Due by Period, Total	1,078.5
Total Debt Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	657.0
Contractual Obligations, Payments Due by Period, 2020	232.5
Contractual Obligations, Payments Due by Period, 2021	202.9
Contractual Obligations, Payments Due by Period, 2022	322.5
Contractual Obligations, Payments Due by Period, 2023	413.1
Contractual Obligations, Payments Due by Period, Thereafter	2,867.5
Contractual Obligations, Payments Due by Period, Total	4,695.5
Operating Lease Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	106.8
Contractual Obligations, Payments Due by Period, 2020	92.0
Contractual Obligations, Payments Due by Period, 2021	78.8
Contractual Obligations, Payments Due by Period, 2022	61.1
Contractual Obligations, Payments Due by Period, 2023	44.3
Contractual Obligations, Payments Due by Period, Thereafter	87.4
Contractual Obligations, Payments Due by Period, Total	470.4
Less Sublease Arrangements [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	(0.8)
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, Thereafter	(1.0)
Contractual Obligations, Payments Due by Period, Total	(3.6)
Outstanding Purchase Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	32.1
Contractual Obligations, Payments Due by Period, 2020	14.6
Contractual Obligations, Payments Due by Period, 2021	11.3
Contractual Obligations, Payments Due by Period, 2022	2.1
Contractual Obligations, Payments Due by Period, Total	60.1
Total Contractual Obligations [Member]
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]
Contractual Obligations, Payments Due by Period, 2019	795.1
Contractual Obligations, Payments Due by Period, 2020	338.5
Contractual Obligations, Payments Due by Period, 2021	292.4
Contractual Obligations, Payments Due by Period, 2022	385.4
Contractual Obligations, Payments Due by Period, 2023	457.1
Contractual Obligations, Payments Due by Period, Thereafter	2,953.9
Contractual Obligations, Payments Due by Period, Total	$ 5,222.4

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, sublease agreement, 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

No definition available.

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation sublease 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

No definition available.

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation sublease 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

No definition available.

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation sublease 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

No definition available.

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation sublease 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

No definition available.

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation, sublease 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

No definition available.

+ Details

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

- Definition

Amount of the recorded unconditional purchase obligation, sublease 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

No definition available.

+ Details

Name:	ajg_RecordedUnconditionalPurchaseObligationSubleaseDueInThirdYear
Namespace Prefix:	ajg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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://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: