Investments - Clean Coal Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified
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3 Months Ended |
12 Months Ended |
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Dec. 31, 2014
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Sep. 30, 2014
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Jun. 30, 2014
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Mar. 31, 2014
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Dec. 31, 2013
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Sep. 30, 2013
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Jun. 30, 2013
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Mar. 31, 2013
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Dec. 31, 2014
Plant
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Dec. 31, 2013
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Dec. 31, 2012
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Sep. 01, 2013
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Mar. 01, 2014
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Mar. 01, 2013
Plant
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Schedule of Equity Method Investments [Line Items] |
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Total revenues |
$ 1,245.4us-gaap_Revenues |
$ 1,286.8us-gaap_Revenues |
$ 1,179.3us-gaap_Revenues |
$ 915.0us-gaap_Revenues |
$ 890.2us-gaap_Revenues |
$ 835.8us-gaap_Revenues |
$ 779.5us-gaap_Revenues |
$ 674.1us-gaap_Revenues |
$ 4,626.5us-gaap_Revenues |
$ 3,179.6us-gaap_Revenues |
$ 2,520.3us-gaap_Revenues |
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Total expenses |
(1,219.1)us-gaap_BenefitsLossesAndExpenses |
(1,199.1)us-gaap_BenefitsLossesAndExpenses |
(1,072.2)us-gaap_BenefitsLossesAndExpenses |
(868.7)us-gaap_BenefitsLossesAndExpenses |
(840.9)us-gaap_BenefitsLossesAndExpenses |
(750.3)us-gaap_BenefitsLossesAndExpenses |
(682.1)us-gaap_BenefitsLossesAndExpenses |
(631.8)us-gaap_BenefitsLossesAndExpenses |
4,359.1us-gaap_BenefitsLossesAndExpenses |
2,905.1us-gaap_BenefitsLossesAndExpenses |
2,275.0us-gaap_BenefitsLossesAndExpenses |
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Ownership interest acquired in a limited liability company |
50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage |
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50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage |
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99.00%us-gaap_EquityMethodInvestmentOwnershipPercentage |
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Amount of promissory note |
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10.0ajg_PromissoryNotesIssuedPrincipalAmount |
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Purchase price paid in cash |
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4.0ajg_CashPurchasePriceOfLimitedLiabilityCompanyLLC |
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Number of clean coal production plants seeking and negotiating for long term production contract |
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8ajg_NumberOfCleanCoalProductionPlantsSeekingAndNegotiatingForLongTermProductionContract |
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Limited Liability Company [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Carrying value of prior non-controlling interest in limited liability company on acquisition date |
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15.6ajg_CarryingValueOfPriorNoncontrollingInterestOnAcquisitionDate / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_LimitedLiabilityCompanyMember |
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Ownership interest acquired in a limited liability company |
50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_LimitedLiabilityCompanyMember |
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50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_LimitedLiabilityCompanyMember |
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Notes Payable, Other Payables [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Interest rate on promissory note |
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3.00%ajg_PromissoryNoteInterestRate / us-gaap_ShortTermDebtTypeAxis = us-gaap_NotesPayableOtherPayablesMember |
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2009 Era Plants [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of additional plants acquired |
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7ajg_NumberOfPlantsWithAdditionalOwnershipInterestAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
12ajg_NumberOfPlantsWithAdditionalOwnershipInterestAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Number of additional plants acquired with ownership interest range one |
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9ajg_NumberOfPlantsWithAdditionalOwnershipInterestRangeOneAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Number of additional plants acquired with ownership interest range two |
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3ajg_NumberOfPlantsWithAdditionalOwnershipInterestRangeTwoAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Ownership interest before additional nine plants acquired |
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24.50%ajg_PreviousOwnershipInterestInPlantWithOwnershipInterestRangeOne / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Ownership interest after additional nine plants acquired |
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49.50%ajg_OwnershipInterestInPlantWithOwnershipInterestRangeOne / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Ownership interest before additional three plants acquired |
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25.00%ajg_PreviousOwnershipInterestInPlantWithOwnershipInterestRangeTwo / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Ownership interest after additional three plants acquired |
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60.00%ajg_OwnershipInterestInPlantWithOwnershipInterestRangeTwo / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
Total revenues |
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260.9us-gaap_Revenues / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
128.3us-gaap_Revenues / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Total expenses |
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264.3us-gaap_BenefitsLossesAndExpenses / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
133.5us-gaap_BenefitsLossesAndExpenses / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Purchase of consideration received |
8.0ajg_PromissoryNoteReceivedAsConsiderationForPurchaseOfInterestInPlants / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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8.0ajg_PromissoryNoteReceivedAsConsiderationForPurchaseOfInterestInPlants / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Payment of cash and other consideration |
5.0ajg_AdditionalPaymentAsConsiderationForPurchaseOfInterestInPlants / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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5.0ajg_AdditionalPaymentAsConsiderationForPurchaseOfInterestInPlants / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Gain on purchase of additional interest |
11.4ajg_GainLossOnPurchaseOfAdditionalInterestUnderEquityMethod / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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11.4ajg_GainLossOnPurchaseOfAdditionalInterestUnderEquityMethod / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Prior percentage of equity interest in limited liability company |
25.00%ajg_PercentageOfEquityInterestHeldByRelatedPartyInAcquiredEntityPriorToAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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25.00%ajg_PercentageOfEquityInterestHeldByRelatedPartyInAcquiredEntityPriorToAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Additional percentage of equity interest in limited liability company upon acquisition |
35.00%ajg_EquityMethodsInvestmentAdditionalOwnershipInterestAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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35.00%ajg_EquityMethodsInvestmentAdditionalOwnershipInterestAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Fixed assets and other amortizable intangible assets acquired upon acquisition |
26.3ajg_FixedAssetsAndOtherAmortizableIntangibleAssetsAcquiredOnAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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26.3ajg_FixedAssetsAndOtherAmortizableIntangibleAssetsAcquiredOnAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Carrying value of prior non-controlling interest in limited liability company on acquisition date |
4.8ajg_CarryingValueOfPriorNoncontrollingInterestOnAcquisitionDate / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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4.8ajg_CarryingValueOfPriorNoncontrollingInterestOnAcquisitionDate / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Other assets acquired upon acquisition |
6.8ajg_OtherAssetsAcquiredOnAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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6.8ajg_OtherAssetsAcquiredOnAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Ownership interest before additional seven plants acquired |
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49.50%ajg_PreviousOwnershipInterestInPlantWithOwnershipInterestRangeThree / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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Ownership interest after additional seven plants acquired |
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100.00%ajg_OwnershipInterestInPlantWithOwnershipInterestRangeThree / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember |
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2011 Era Plants [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of additional plants acquired |
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5ajg_NumberOfPlantsWithAdditionalOwnershipInterestAcquired / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Total revenues |
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381.6us-gaap_Revenues / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Total expenses |
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405.7us-gaap_BenefitsLossesAndExpenses / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Gain on purchase of additional interest |
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25.6ajg_GainLossOnPurchaseOfAdditionalInterestUnderEquityMethod / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Fixed assets and other amortizable intangible assets acquired upon acquisition |
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26.3ajg_FixedAssetsAndOtherAmortizableIntangibleAssetsAcquiredOnAcquisition / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Ownership interest before additional one plant acquired |
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48.80%ajg_PreviousOwnershipInterestInPlantWithOwnershipInterestRangeFour / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Ownership interest after additional one plant acquired |
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90.00%ajg_OwnershipInterestInPlantWithOwnershipInterestRangeFour / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Ownership interest before additional three plants acquired |
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49.00%ajg_PreviousOwnershipInterestInPlantWithOwnershipInterestRangeFive / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Ownership interest after additional three plants acquired |
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100.00%ajg_OwnershipInterestInPlantWithOwnershipInterestRangeFive / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Ownership interest before additional one plant acquired |
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98.00%ajg_PreviousOwnershipInterestInPlantWithOwnershipInterestRangeSix / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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Ownership interest after additional one plant acquired |
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100.00%ajg_OwnershipInterestInPlantWithOwnershipInterestRangeSix / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember |
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2011 Era Plants [Member] | Notes Payable, Other Payables [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Total revenues |
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84.0us-gaap_Revenues / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember / us-gaap_ShortTermDebtTypeAxis = us-gaap_NotesPayableOtherPayablesMember |
33.7us-gaap_Revenues / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember / us-gaap_ShortTermDebtTypeAxis = us-gaap_NotesPayableOtherPayablesMember |
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Total expenses |
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93.0us-gaap_BenefitsLossesAndExpenses / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember / us-gaap_ShortTermDebtTypeAxis = us-gaap_NotesPayableOtherPayablesMember |
36.9us-gaap_BenefitsLossesAndExpenses / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember / us-gaap_ShortTermDebtTypeAxis = us-gaap_NotesPayableOtherPayablesMember |
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Chem-Mod LLC [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of clean coal production plants owned |
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34ajg_NumberOfCleanCoalProductionPlantsOwned / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_ChemModLlcMember |
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Chem-Mod LLC [Member] | 2009 Era Plants [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of clean coal production plants owned |
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14ajg_NumberOfCleanCoalProductionPlantsOwned / us-gaap_InvestmentTypeAxis = ajg_TwoThousandNineEraPlantsMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_ChemModLlcMember |
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Chem-Mod LLC [Member] | 2011 Era Plants [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of clean coal production plants owned |
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20ajg_NumberOfCleanCoalProductionPlantsOwned / us-gaap_InvestmentTypeAxis = ajg_TwoThousandElevenEraPlantsMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_ChemModLlcMember |
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Long Term Production Contracts [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of clean coal production plants owned |
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26ajg_NumberOfCleanCoalProductionPlantsOwned / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_LongTermProductionContractsMember |
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Clean-Coal Investments [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Number of variable interest entities |
1ajg_NumberOfVariableInterestEntities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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1ajg_NumberOfVariableInterestEntities / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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Clean-Coal Investments [Member] | VIE [Member] |
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Schedule of Equity Method Investments [Line Items] |
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Total assets of limited liability companies |
4.4ajg_EquityMethodInvestmentAssetsOfLimitedLiabilityCompanies / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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4.4ajg_EquityMethodInvestmentAssetsOfLimitedLiabilityCompanies / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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Total liability of limited liability companies |
1.4ajg_EquityMethodInvestmentLiabilitiesOfLimitedLiabilityCompanies / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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1.4ajg_EquityMethodInvestmentLiabilitiesOfLimitedLiabilityCompanies / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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Total revenues of limited liability companies |
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33.4ajg_EquityMethodInvestmentRevenuesOfLimitedLiabilityCompanies / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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Total expenses of limited liability companies |
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$ 41.0ajg_EquityMethodInvestmentExpensesOfLimitedLiabilityCompanies / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = ajg_CleanCoalInvestmentsMember |
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