XML 91 R77.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Restricted Stock, Performance Share and Cash Awards - Additional Information (Detail) |  |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 16, 2017 USD ($) shares | Mar. 17, 2016 USD ($) shares | Mar. 11, 2015 USD ($) shares | Mar. 31, 2017 shares | Mar. 31, 2016 shares | Mar. 31, 2015 shares | Dec. 31, 2017 USD ($) Times shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares | Dec. 31, 2014 USD ($) shares | Mar. 12, 2014 USD ($) shares | May 12, 2009 shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation, shares outstanding | shares |  |  |  |  |  |  | 10,600,000 | 10,300,000 | 8,800,000 | 8,400,000 |  |
Shares granted in the period | shares |  |  |  |  |  |  | 476,350 | 479,167 | 394,975 |  |  |
Long Term Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share-based compensation, shares outstanding, Value |  |  |  |  |  |  | $ 4,000,000 |  |  |  |  |
Share-based compensation, shares outstanding | shares |  |  |  |  |  |  | 4,000,000 |  |  |  |  |
Shares granted in the period | shares |  |  |  |  |  |  | 4,000,000 |  |  |  |  |
Restricted Stock Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Shares authorized | shares |  |  |  |  |  |  |  |  |  |  |  | 4,000,000
Shares granted in the period | shares |  |  |  | 477,500 | 466,600 | 362,600 |  |  |  |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Shares granted in the period | shares |  |  |  |  |  |  | 476,350 | 479,167 | 394,975 |  |  |
Fair value of grants in period |  |  |  |  |  |  | $ 26,800,000 | $ 20,400,000 | $ 16,700,000 |  |  |
Restricted stock or unit expense |  |  |  |  |  |  | $ 19,600,000 | $ 18,200,000 | $ 14,400,000 |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Share based payment award vesting date |  |  |  | Mar. 19, 2022 | Mar. 17, 2021 | Mar. 11, 2020 |  |  |  |  |  |
Restricted Stock Units (RSUs) [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Period of service from grant date stock options awarded not subject to forfeiture |  |  |  |  |  |  | 2 years | 2 years | 2 years |  |  |
Restricted Stock Units (RSUs) [Member] | Executive Officer [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Minimum age of employee with not subject to award forfeiture on condition compliance |  |  |  |  |  |  | 55 years | 55 years | 55 years |  |  |
Unvested Restricted Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total intrinsic value |  |  |  |  |  |  | $ 109,300,000 | $ 80,000,000 |  |  |  |
Equity awards with an aggregate fair value |  |  |  |  |  |  | $ 23,300,000 | 14,200,000 |  |  |  |
Cash Awards [Member] | Officer and Key Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | The ultimate award value will be equal to the trailing twelve-month price of our common stock on December 31, 2019, multiplied by the number of units subject to the award, but limited to between 0.5 and 1.5 times the original value of the units determined as of the grant date. |  |  |  |  |
Performance awards period, years |  |  |  |  |  |  | 1 year |  |  |  |  |
Vesting period, years |  |  |  |  |  |  | 2 years |  |  |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value | $ 14,300,000 |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares | 255,000 |  |  |  |  |  |  |  |  |  |  |
Ultimate award value, multiples of original value of the units, minimum | Times |  |  |  |  |  |  | 0.5 |  |  |  |  |
Ultimate award value, multiples of original value of the units, maximum | Times |  |  |  |  |  |  | 1.5 |  |  |  |  |
2017 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  | 242,000 |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 0 |  |  |  |  |
Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Shares authorized | shares | 86,250 | 72,900 | 53,900 |  |  |  |  |  |  |  |  |
Total intrinsic value |  |  |  |  |  |  | $ 10,700,000 |  |  |  |  |
Performance unit awards approved, Fair value | $ 4,900,000 | $ 3,200,000 | $ 2,500,000 |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | Granted units for the 2017 provisional awards will fully vest based on continuous employment through March 16, 2020 and will be settled in shares of our common stock on a one-for-one basis as soon as practicable thereafter. |  |  |  |  |
Performance awards period, years |  |  |  |  |  |  | 1 year |  |  |  |  |
Vesting period, years |  |  |  |  |  |  | 2 years |  |  |  |  |
Compensation expense |  |  |  |  |  |  | $ 3,300,000 | 2,900,000 |  |  |  |
2017 Performance Share Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional awards, terms |  |  |  |  |  |  | The 2017 awards are subject to a three-year performance period that begins on January 1, 2017, and vest on the three-year anniversary of the date of grant (March 16, 2020). |  |  |  |  |
Performance awards period, years |  |  |  |  |  |  | 3 years |  |  |  |  |
Vesting period, years |  |  |  |  |  |  | 3 years |  |  |  |  |
Performance awards expiration date |  |  |  |  |  |  | Mar. 16, 2020 |  |  |  |  |
2016 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 10,600,000 |  |  |  |  |
2016 Provisional Cash Awards [Member] | Officer and Key Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value |  | $ 17,400,000 |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  | 397,000 |  | 385,000 |  |  |  |  |  |  |  |
2015 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  | $ 9,300,000 | 6,600,000 |  |  |  |
2015 Provisional Cash Awards [Member] | Officer and Key Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value |  |  | $ 14,600,000 |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  | 315,000 |  | 294,000 |  |  |  |  |  |  |
2014 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  | 199,000 |  |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  |  | $ 4,500,000 | $ 4,900,000 | $ 0 |  |
Grants vested in period fair value |  |  |  |  |  |  | $ 9,300,000 |  |  |  |  |
2014 Provisional Cash Awards [Member] | Officer and Key Employees [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash awards approved for future grant by compensation committee, value |  |  |  |  |  |  |  |  |  |  | $ 10,800,000 |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  | 220,000 |  |  |  |  | 229,000 |
2012 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  |  |  | 342,000 |  |  |
Grants vested in period fair value |  |  |  |  |  |  |  |  | $ 15,800,000 |  |  |
2011 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  |  |  |  | 411,000 |  |
Grants vested in period fair value |  |  |  |  |  |  |  |  |  | $ 17,600,000 |  |
2013 Provisional Cash Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Provisional compensation cash award approved for future grant by compensation committee, units | shares |  |  |  |  |  |  |  | 246,000 |  |  |  |
Cash-based compensation awards, expenses |  |  |  |  |  |  |  | $ 11,200,000 |  |  |  |

X
- Definition Age Of Employee No Longer Subject To Award Forfeiture On Condition Compliance + References No definition available. + Details
Name: | ajg_AgeOfEmployeeNoLongerSubjectToAwardForfeitureOnConditionCompliance
Namespace Prefix: | ajg_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cash based compensation award, expenses + References No definition available. + Details
Name: | ajg_CashBasedCompensationAwardExpenses
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] + References No definition available. + Details
Name: | ajg_CommonStockOptionsRestrictedStockWarrantsAndChangesInCapitalizationLineItems
Namespace Prefix: | ajg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period Of Service From Grant Date Stock Options Awarded Not Subject To Forfeiture + References No definition available. + Details
Name: | ajg_PeriodOfServiceFromGrantDateStockOptionsAwardedNotSubjectToForfeiture
Namespace Prefix: | ajg_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Provisional compensation cash award approved for future grant by compensation committee, units + References No definition available. + Details
Name: | ajg_ProvisionalCompensationCashAwardApprovedForFutureGrantByCompensationCommitteeUnits
Namespace Prefix: | ajg_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Provisional Compensation Cash Award Approved for Future Grant by Compensation Committee, Value + References No definition available. + Details
Name: | ajg_ProvisionalCompensationCashAwardApprovedForFutureGrantByCompensationCommitteeValue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Date + References No definition available. + Details
Name: | ajg_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingDate
Namespace Prefix: | ajg_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Option Nonvested Intrinsic Value + References No definition available. + Details
Name: | ajg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionNonvestedIntrinsicValue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Fair Value + References No definition available. + Details
Name: | ajg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodFairValue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Value + References No definition available. + Details
Name: | ajg_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingValue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Fair Value + References No definition available. + Details
Name: | ajg_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingFairValue
Namespace Prefix: | ajg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Ultimate Award Value Multiples of Original Value of Units, Maximum + References No definition available. + Details
Name: | ajg_UltimateAwardValueMultiplesOfOriginalValueOfUnitsMaximum
Namespace Prefix: | ajg_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition Ultimate Award Value Multiples of Original Value of Units, Minimum + References No definition available. + Details
Name: | ajg_UltimateAwardValueMultiplesOfOriginalValueOfUnitsMinimum
Namespace Prefix: | ajg_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of plan compensation cost recognized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65895303&loc=d3e17540-113929 + Details
Name: | us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
Name: | us-gaap_RestrictedStockExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target 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 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date the equity-based award expires, in CCYY-MM-DD format. + 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)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpirationDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
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 Number of options outstanding, including both vested and non-vested options. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Description of pertinent provisions of equity-based compensation awards that have actual or potential impact upon the company's financial statements. + 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) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + 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)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
Name: | us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Aggregate value of stock related to Restricted Stock Awards issued during the period. + References Reference 1: 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(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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=27012166&loc=d3e187085-122770 Reference 4: 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(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_PlanNameAxis=ajg_LongTermIncentivePlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_RestrictedStockPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_RestrictedStockUnitsRsusMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_TitleOfIndividualAxis=us-gaap_ExecutiveOfficerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_UnearnedRestrictedStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_CashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TitleOfIndividualAxis=us-gaap_OfficerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandSeventeenProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandSeventeenPerformanceShareAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandSixteenProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandFifteenProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandFourteenProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandTwelveProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandElevenProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=ajg_TwoThousandThirteenProvisionalCashAwardsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |