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Commitments, Contingencies and Off-Balance Sheet Arrangements - Off-Balance Sheet Commitments (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]  
Amount of Commitment Expiration by Period - 2015 $ 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearOne
Amount of Commitment Expiration by Period - 2016 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearTwo
Amount of Commitment Expiration by Period - 2017 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThree
Amount of Commitment Expiration by Period - 2018 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFour
Amount of Commitment Expiration by Period - 2019 0.9ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFive
Amount of Commitment Expiration by Period - Thereafter 42.0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThereafter
Total Amounts Committed 46.1ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitments
Letters of Credit [Member]  
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]  
Amount of Commitment Expiration by Period - 2015 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearOne
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Amount of Commitment Expiration by Period - 2016 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearTwo
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Amount of Commitment Expiration by Period - 2017 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThree
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Amount of Commitment Expiration by Period - 2018 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFour
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Amount of Commitment Expiration by Period - 2019 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFive
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Amount of Commitment Expiration by Period - Thereafter 22.6ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThereafter
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Total Amounts Committed 22.6ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitments
/ us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis
= ajg_LettersOfCreditMember
Financial Guarantees [Member]  
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]  
Amount of Commitment Expiration by Period - 2015 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearOne
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Amount of Commitment Expiration by Period - 2016 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearTwo
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Amount of Commitment Expiration by Period - 2017 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThree
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Amount of Commitment Expiration by Period - 2018 0.8ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFour
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Amount of Commitment Expiration by Period - 2019 0.9ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFive
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Amount of Commitment Expiration by Period - Thereafter 16.5ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThereafter
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Total Amounts Committed 20.6ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_FinancialGuaranteeMember
Funding Commitments [Member]  
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items]  
Amount of Commitment Expiration by Period - 2015 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearOne
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember
Amount of Commitment Expiration by Period - 2016 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearTwo
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember
Amount of Commitment Expiration by Period - 2017 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThree
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember
Amount of Commitment Expiration by Period - 2018 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFour
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember
Amount of Commitment Expiration by Period - 2019 0ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearFive
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember
Amount of Commitment Expiration by Period - Thereafter 2.9ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitmentsWithinYearThereafter
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember
Total Amounts Committed $ 2.9ajg_AmountOfCommitmentExpirationOffBalanceSheetCommitments
/ us-gaap_GuaranteeObligationsByNatureAxis
= ajg_FundingCommitmentsMember