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Other Current Assets - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] |
Collateralized delinquency period of receivables | 7 days

X
- Definition Collateralized Delinquency Receivables Period + References No definition available. + Details
Name: | ajg_CollateralizedDelinquencyReceivablesPeriod
Namespace Prefix: | ajg_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration