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Earnings Per Share - Additional information (Detail) - shares shares in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Options to purchase common stock shares outstanding	0.3	4.3

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- Definition

Options To Purchase Common Stock Shares Outstanding

+ References

No definition available.

+ Details

Name:	ajg_OptionsToPurchaseCommonStockSharesOutstanding
Namespace Prefix:	ajg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration