XML 64 R50.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Summary of Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) Country Segment shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Number of reportable operating segments | Segment | 3 |  |
Number of commercial clean coal production facilities | 34 |  |
Number of countries in which the company has operations | Country | 32 |  |
Number of countries in which the company does business through a network of correspondent brokers and consultants | Country | 150 |  |
Percentage of ownership interest | 50.00% |  |
Percentage of variable ownership interest | 50.00% |  |
Description of contracts renewal period | One year or less |  |
Maturity period of short-term investments | 90 days or less |  |
Allowances for estimated policy cancellations | $ 7,400,000 | $ 7,100,000 |
Allowance for doubtful accounts | $ 13,500,000 | 12,800,000 |
Premium financing contracts, terms | Premium financing contracts are structured to minimize potential bad debt expense to us. Such receivables are generally considered delinquent after seven days of the payment due date. In normal course, insurance policies are cancelled within one month of the contractual payment due date if the payment remains delinquent. |  |
Outstanding loan receivable | $ 305,500,000 | 241,200,000 |
Write-off of amortizable intangible assets | $ 6,200,000 | $ 1,800,000 | $ 11,500,000
Minimum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Percentage of ownership interest | 1.00% |  |
Maximum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Percentage of ownership interest | 50.00% |  |
Employee Stock Purchase Plan [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Shares authorized | shares | 8,000,000 |  |
Percentage of employees contribution | 15.00% |  |
Purchase price of common stock, percentage | 95.00% |  |
Aggregate fair market value of shares purchased | $ 25,000 |  |
Maximum number of shares purchased by employees | shares | 2,000 |  |
Shares available for grant | shares | 7,200,000 |  |
Expiration Lists [Member] | Minimum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of intangibles assets, years | 3 years |  |
Expiration Lists [Member] | Maximum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of intangibles assets, years | 15 years |  |
Non-Compete Agreements [Member] | Minimum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of intangibles assets, years | 3 years |  |
Non-Compete Agreements [Member] | Maximum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of intangibles assets, years | 5 years |  |
Trade Names [Member] | Minimum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of intangibles assets, years | 5 years |  |
Trade Names [Member] | Maximum [Member] |  |  |
Significant Accounting Policies [Line Items] |  |  |
Estimated useful lives of intangibles assets, years | 15 years |  |

X
- Definition Allowances for Estimated Policy Cancellations. + References No definition available. + Details
Name: | ajg_AllowancesForEstimatedPolicyCancellations
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Description of contracts renewal period. + References No definition available. + Details
Name: | ajg_DescriptionOfContractsRenewalPeriod
Namespace Prefix: | ajg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Premium financing contracts are structured to minimize potential bad debt expense. Such receivables are generally considered delinquent after seven days of the payment due date. + References No definition available. + Details
Name: | ajg_InsuranceContractsPremiumFinancingTerms
Namespace Prefix: | ajg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maturity Period Of Short Term Investments. + References No definition available. + Details
Name: | ajg_MaturityPeriodOfShortTermInvestments
Namespace Prefix: | ajg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of commercial clean coal production facilities. + References No definition available. + Details
Name: | ajg_NumberOfCommercialCleanCoalProductionFacilities
Namespace Prefix: | ajg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of Countries in which the Company does Business through a Network of Correspondent Brokers and Consultants + References No definition available. + Details
Name: | ajg_NumberOfCountriesInWhichCompanyDoesBusinessThroughNetworkOfCorrespondentBrokersAndConsultants
Namespace Prefix: | ajg_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of Countries in which the Company has Operations + References No definition available. + Details
Name: | ajg_NumberOfCountriesInWhichCompanyHasOperations
Namespace Prefix: | ajg_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share-Based Compensation Arrangement By Share-Based Payment Award, Aggregate Fair Market Value. + References No definition available. + Details
Name: | ajg_ShareBasedCompensationArrangementByShareBasedPaymentAwardAggregateFairMarketValue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Significant Accounting Policies [Line Items] + References No definition available. + Details
Name: | ajg_SignificantAccountingPoliciesLineItems
Namespace Prefix: | ajg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount billed to customers under long-term contracts or programs but not paid. Excludes amounts due that have been withheld because of retainage provisions in a contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 910 -SubTopic 310 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=68065756&loc=d3e49354-109356 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3(c)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AccountsReceivableBilledForLongTermContractsOrPrograms
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AllowanceForDoubtfulAccountsReceivable
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
Name: | us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
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 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/2003/role/presentationRef -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 Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Discount rate from fair value on offering date that participants pay for shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The highest quantity of shares an employee can purchase under the plan per period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 5A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=77990344&loc=SL6759159-111685 + Details
Name: | us-gaap_VariableInterestEntityOwnershipPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis=ajg_EmployeeStockPurchasePlanMember
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: |