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

X
- 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