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Other Financial Data - (Other Current Assets) (Additional Information) (Details)	3 Months Ended
	Mar. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Collateralized delinquency period of receivables	7 days

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- 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