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Commitments, Contingencies and Off-Balance Sheet Arrangements - Litigation, Regulatory and Taxation Matters- Additional Information (Detail) | 3 Months Ended
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Mar. 31, 2018 Companies
Minimum [Member] |
Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] |
Number of micro-captive insurance companies organized or managed | 100

X
- Definition Commitments Contingencies And Off Balance Sheet Arrangements [Line Items] + References No definition available. + Details
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Namespace Prefix: | ajg_
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X
- Definition Micro captive Insurance Companies Managed + References No definition available. + Details
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Namespace Prefix: | ajg_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |