XML 54 R43.htm IDEA: XBRL DOCUMENT
v3.21.2

Business Combinations - Additional Information (Detail) shares in Millions |  |  |  |  | 3 Months Ended | 6 Months Ended | 234 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 29, 2021 | May 20, 2021 USD ($) | May 17, 2021 USD ($) shares | May 12, 2021 USD ($) | Jun. 30, 2021 USD ($) Entity shares | Jun. 30, 2020 USD ($) Entity | Jun. 30, 2021 USD ($) Entity | Jun. 30, 2020 USD ($) Entity | Jun. 30, 2021 USD ($) Entity | Dec. 31, 2020 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cash Paid |  |  |  |  |  |  | $ 592,000,000.0 |  |  |
Aggregate principal amount |  |  |  |  | $ 6,114,100,000 |  | 6,114,100,000 |  | $ 6,114,100,000 | $ 4,551,600,000
Accretion of the discount on acquisition |  |  |  |  | 9,200,000 | $ 7,700,000 | 18,200,000 | $ 18,400,000 |  |
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  |  |  |  | $ (4,600,000) | $ (8,000,000.0) | $ (10,600,000) | $ 91,600,000 |  |
Number of companies acquired | Entity |  |  |  |  | 26 | 44 | 48 | 102 | 597 |
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  |  |  |  | $ 1,068,100,000 |  |  |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  |  |  |  | 560,100,000 |  |  |
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  |  |  |  | $ 536,200,000 |  | 536,200,000 |  | $ 536,200,000 |
Aggregate amount of earnout obligation expected settlement in cash |  |  |  |  | 23,900,000 |  | 23,900,000 |  | 23,900,000 |
Goodwill |  |  |  |  | 387,200,000 |  | 387,200,000 |  | 387,200,000 |
Expiration lists |  |  |  |  | 259,500,000 |  | 259,500,000 |  | 259,500,000 |
Non-compete agreements |  |  |  |  | 18,200,000 |  | 18,200,000 |  | 18,200,000 |
Trade names |  |  |  |  | 4,100,000 |  | 4,100,000 |  | 4,100,000 |
Total revenues |  |  |  |  | 1,936,800,000 | $ 1,620,000,000.0 | 4,129,400,000 | $ 3,527,300,000 |  |
Brokerage and Risk Management [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | 387,200,000 |  | 387,200,000 |  | 387,200,000 |
Expiration lists |  |  |  |  | 259,500,000 |  | 259,500,000 |  | 259,500,000 |
Non-compete agreements |  |  |  |  | 18,200,000 |  | 18,200,000 |  | 18,200,000 |
Trade names |  |  |  |  | 4,100,000 |  | 4,100,000 |  | 4,100,000 |
Brokerage [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Impairment wrote off of amortizable assets |  |  |  |  | 8,000,000.0 | $ 200,000 | 13,100,000 | $ 46,000,000.0 |  |
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 138,600,000 |  | 138,600,000 |  | 138,600,000 |
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 16,400,000 |  | 16,400,000 |  | 16,400,000 |
Brokerage [Member] | Trade Names [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | $ 3,300,000 |  | $ 3,300,000 |  | $ 3,300,000 |
Minimum [Member] | Expiration Lists [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  |  | 2 years |  |  |
Minimum [Member] | Non-Compete Agreements [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  |  | 2 years |  |  |
Minimum [Member] | Trade Names [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  |  | 2 years |  |  |
Maximum [Member] | Expiration Lists [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  |  | 15 years |  |  |
Maximum [Member] | Non-Compete Agreements [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  |  | 6 years |  |  |
Maximum [Member] | Trade Names [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated useful lives of intangibles assets, years |  |  |  |  |  |  | 15 years |  |  |
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Periodic payment of interest |  |  |  |  | 2.50% |  | 2.50% |  | 2.50% |
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Periodic payment of interest |  |  |  |  | 3.50% |  | 3.50% |  | 3.50% |
Unsecured Senior Notes [Member] | Private Placement [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 1,500,000,000.0 |  |  | $ 1,500,000,000.0 |  | $ 1,500,000,000.0 |  | $ 1,500,000,000.0 |
Weighted average interest rate |  | 3.31% |  |  |  |  |  |  |  |
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 650,000,000.0 |  |  | 650,000,000.0 |  | 650,000,000.0 |  | 650,000,000.0 |
Periodic payment of interest |  | 2.50% |  |  |  |  |  |  |  |
Periodic payments of interest and principal, expiry year |  | 2031 |  |  |  |  |  |  |  |
Unsecured Senior Notes [Member] | Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Notes redemption date | Aug. 13, 2021 |  |  |  |  |  |  |  |  |
Unsecured Senior Notes [Member] | Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] | Private Placement [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 850,000,000.0 |  |  | $ 850,000,000.0 |  | $ 850,000,000.0 |  | $ 850,000,000.0 |
Periodic payment of interest |  | 3.50% |  |  |  |  |  |  |  |
Periodic payments of interest and principal, expiry year |  | 2051 |  |  |  |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Stock issuance from public offering (in shares) | shares |  |  |  |  | 10.3 |  |  |  |  |
Willis Towers Watson PLC [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Cash Paid |  |  |  | $ 3,570,000,000 |  |  |  |  |  |
Willis Towers Watson PLC [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Agreement termination date | Jul. 29, 2021 |  |  |  |  |  |  |  |  |
Willis Towers Watson PLC [Member] | Common Stock [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Stock issuance from public offering (in shares) | shares |  |  | 10.3 |  |  |  |  |  |  |
Net proceeds after underwriting, discounts and other expense |  |  | $ 1,437,900,000 |  |  |  |  |  |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 5.0 |  | 5.0 |  | 5.0 |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 15.0 |  | 15.0 |  | 15.0 |
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 7.0 |  | 7.0 |  | 7.0 |
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 10.5 |  | 10.5 |  | 10.5 |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 3.0 |  | 3.0 |  | 3.0 |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 4.8 |  | 4.8 |  | 4.8 |
2020 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  | 4.30% |  |  |
2020 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Attrition rate |  |  |  |  |  |  | 16.10% |  |  |
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 10.0 |  | 10.0 |  | 10.0 |
2020 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Measurement input |  |  |  |  | 13.5 |  | 13.5 |  | 13.5 |
Business Acquisition [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Trade names |  |  |  |  | $ 4,100,000 |  | $ 4,100,000 |  | $ 4,100,000 |
Annualized revenue of business acquisitions |  |  |  |  |  |  | 173,800,000 |  |  |
Total revenues |  |  |  |  |  |  | 60,600,000 |  |  |
Net earnings |  |  |  |  |  |  | 2,700,000 |  |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Expiration lists |  |  |  |  | 259,500,000 |  | 259,500,000 |  | 259,500,000 |
Non-compete agreements |  |  |  |  | 18,200,000 |  | 18,200,000 |  | 18,200,000 |
Deferred tax liability |  |  |  |  | $ 26,500,000 |  | $ 26,500,000 |  | $ 26,500,000 |

X
- Definition Agreement termination date. + References No definition available. + Details
Name: | ajg_AgreementTerminationDate
Namespace Prefix: | ajg_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Business Acquisition Purchase Price Allocation Goodwill Not Expected Tax Deductible Amount + References No definition available. + Details
Name: | ajg_BusinessAcquisitionPurchasePriceAllocationGoodwillNotExpectedTaxDeductibleAmount
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business Acquisitions Purchase Price Allocation Goodwill Amount + References No definition available. + Details
Name: | ajg_BusinessAcquisitionsPurchasePriceAllocationGoodwillAmount
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Business combination aggregate earnout obligation expected settlement in cash. + References No definition available. + Details
Name: | ajg_BusinessCombinationAggregateEarnoutObligationExpectedSettlementInCash
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business combination aggregate earnout obligation expected settlement in cash or stock at option. + References No definition available. + Details
Name: | ajg_BusinessCombinationAggregateEarnoutObligationExpectedSettlementInCashOrStockAtOption
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Business Combination ,Aggregate Earnout Obligation, Maximum + References No definition available. + Details
Name: | ajg_BusinessCombinationAggregateEarnoutObligationMaximum
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Business Combination Aggregate Earnout Obligation Recorded Maximum + References No definition available. + Details
Name: | ajg_BusinessCombinationAggregateEarnoutObligationRecordedMaximum
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Business Combination, Annualize Revenue + References No definition available. + Details
Name: | ajg_BusinessCombinationAnnualizeRevenue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Debt Instrument Maturity Year + References No definition available. + Details
Name: | ajg_DebtInstrumentMaturityYear
Namespace Prefix: | ajg_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument redemption date. + References No definition available. + Details
Name: | ajg_DebtInstrumentRedemptionDate
Namespace Prefix: | ajg_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt long-term and short-term combined amount gross. + References No definition available. + Details
Name: | ajg_DebtLongTermAndShortTermCombinedAmountGross
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Expiration lists + References No definition available. + Details
Name: | ajg_ExpirationLists
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair Value Inputs, Attrition Rate + References No definition available. + Details
Name: | ajg_FairValueInputsAttritionRate
Namespace Prefix: | ajg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of decrease in additional paid in capital (APIC) resulting from direct costs associated with issuing stock. Includes, but is not limited to, legal and accounting fees and direct costs associated with stock issues under a shelf registration. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 + Details
Name: | us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Value of input used to measure alternative investment. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_AlternativeInvestmentMeasurementInput
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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
- Definition Amount after tax of pro forma income from continuing operations as if the business combination had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessAcquisitionsProFormaIncomeLossFromContinuingOperationsBeforeChangesInAccountingAndExtraordinaryItemsNetOfTax
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessAcquisitionsProFormaRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition For transactions that are recognized separately from the acquisition of assets and assumptions of liabilities in the business combination, such as related to pre-existing relationships with the acquiree, this element represents the disclosure of the amount of acquisition related costs which have been expensed. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessCombinationSeparatelyRecognizedTransactionsAdditionalDisclosuresAcquisitionCostExpensed
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted average interest rate of debt outstanding. + References No definition available. + Details
Name: | us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of deferred tax liability attributable to taxable temporary differences. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(26)(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(15)(b)(2)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(15)(2)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(26)(a)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DeferredIncomeTaxLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Useful life of finite-lived intangible assets, 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_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Gross carrying amount before accumulated amortization as of the balance sheet date of payments made to third parties in exchange for their agreement not to engage in specified competitive practices in specified geographic territories for a specified period of time. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
Name: | us-gaap_FiniteLivedNoncompeteAgreementsGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Gross carrying amount before accumulated amortization as of the balance sheet date of the rights acquired through registration of a trade name to gain or protect exclusive use thereof for a reasonably expected period of economic benefit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
Name: | us-gaap_FiniteLivedTradeNamesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275 + Details
Name: | us-gaap_ImpairmentOfIntangibleAssetsFinitelived
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The number of businesses acquired by the entity during the period. + References No definition available. + Details
Name: | us-gaap_NumberOfBusinessesAcquired
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585 + Details
Name: | us-gaap_PaymentsToAcquireBusinessesGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of new stock issued during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ajg_InsuranceBrokerageAndRiskManagementServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ajg_BrokerageSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=ajg_ExpirationListsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TradeNamesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ExtinguishmentOfDebtAxis=ajg_FixedRateOfTwoPointFiveZeroPercentBalloonDueMayTwentyTwoThousandThirtyOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ExtinguishmentOfDebtAxis=ajg_FixedRateOfThreePointFiveZeroPercentBalloonDueMayTwentyTwoThousandFiftyOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=ajg_SeniorUnsecuredNoteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=us-gaap_PrivatePlacementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=ajg_WillisTowersWatsonPLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=ajg_TwoThousandTwentyOneAcquisitionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=us-gaap_IncomeApproachValuationTechniqueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=ajg_TwoThousandTwentyAcquisitionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputLongTermRevenueGrowthRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=ajg_BusinessAcquisitionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |