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Income Taxes - Deferred Tax Assets and Liabilities (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Income Tax Disclosure [Abstract]    
Alternative minimum tax and other credit carryforwards $ 233.4ajg_DeferredTaxAssetsAlternativeMinimumTaxAndOtherCreditCarryforwards $ 147.4ajg_DeferredTaxAssetsAlternativeMinimumTaxAndOtherCreditCarryforwards
Accrued and unfunded compensation and employee benefits 166.9ajg_DeferredTaxAssetsAccruedAndUnfundedCompensationAndEmployeeBenefits 136.6ajg_DeferredTaxAssetsAccruedAndUnfundedCompensationAndEmployeeBenefits
Amortizable intangible assets 66.7us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets 12.5us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets
Compensation expense related to stock options 14.7us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost 14.2us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost
Accrued liabilities 31.0us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities 30.2us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities
Accrued pension liability 24.3ajg_DeferredTaxAssetsAccruedPensionLiability 9.2ajg_DeferredTaxAssetsAccruedPensionLiability
Investments 10.0us-gaap_DeferredTaxAssetsInvestments 9.0us-gaap_DeferredTaxAssetsInvestments
Net operating loss carryforwards 10.0us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 11.9us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
Deferred rent liability 8.5us-gaap_DeferredRentCreditCurrent 8.2us-gaap_DeferredRentCreditCurrent
Other 4.8us-gaap_DeferredTaxAssetsOther 6.8us-gaap_DeferredTaxAssetsOther
Total deferred tax assets 570.3us-gaap_DeferredTaxAssetsGross 386.0us-gaap_DeferredTaxAssetsGross
Valuation allowance for deferred tax assets (75.5)us-gaap_DeferredTaxAssetsValuationAllowance (21.3)us-gaap_DeferredTaxAssetsValuationAllowance
Deferred tax assets 494.8us-gaap_DeferredTaxAssetsNet 364.7us-gaap_DeferredTaxAssetsNet
Nondeductible amortizable intangible assets 338.7us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets 184.0us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets
Investment-related partnerships 26.6ajg_DeferredTaxLiabilitiesInvestmentRelatedPartnerships 13.2ajg_DeferredTaxLiabilitiesInvestmentRelatedPartnerships
Depreciable fixed assets 8.8ajg_DeferredTaxLiabilitiesDepreciableFixedAssets 5.2ajg_DeferredTaxLiabilitiesDepreciableFixedAssets
Other prepaid items 4.3ajg_DeferredTaxLiabilitiesOtherPrepaidItems 4.7ajg_DeferredTaxLiabilitiesOtherPrepaidItems
Accrued liabilities   2.4ajg_DeferredTaxLiabilitiesAccruedLiabilities
Total deferred tax liabilities 378.4us-gaap_DeferredTaxLiabilities 209.5us-gaap_DeferredTaxLiabilities
Net deferred tax assets $ 116.4us-gaap_DeferredTaxAssetsLiabilitiesNet $ 155.2us-gaap_DeferredTaxAssetsLiabilitiesNet