XML 116 R102.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Investments - Other Investments - Additional Information (Detail) | Dec. 31, 2018 USD ($) Investment Venture | Dec. 31, 2017 USD ($)
--- | --- | ---
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 59,500,000 | $ 79,100,000
Non controlling interest certified low-income housing developments | Investment | 8 |
Other Investments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 5,000,000 | $ 3,800,000
Other Investments [Member] | Four Venture Capital Funds [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 5,000,000 |
Non controlling interest in venture capital funds number | Venture | 4 |
Other Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Assets | $ 15,000,000 |
Liabilities | 0 |
Other Investments [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Eight Certified Low Income Housing Developments [Member] |  |
Schedule of Equity Method Investments [Line Items] |  |
Carrying value of investments | $ 0 |

X
- Definition Carrying Value of Investments + References No definition available. + Details
Name: | ajg_CarryingValueOfInvestments
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number Of Certified Low Income Housing Developments Investment + References No definition available. + Details
Name: | ajg_NumberOfCertifiedLowIncomeHousingDevelopmentsInvestment
Namespace Prefix: | ajg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number Of Venture Capital Funds Investment + References No definition available. + Details
Name: | ajg_NumberOfVentureCapitalFundsInvestment
Namespace Prefix: | ajg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount of assets reported by an equity method investment of the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33912-111571 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(g)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612 + Details
Name: | us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of liabilities reported by an equity method investment of the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33912-111571 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(g)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760 + Details
Name: | us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=ajg_VentureCapitalFundsAndOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | invest_InvestmentAxis=ajg_FourVentureCapitalFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | invest_InvestmentAxis=ajg_EightCertifiedLowIncomeHousingDevelopmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |