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Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2017
Construction Risk Solutions, LLC [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Jan. 01, 2017
Hill, Chesson And Woody [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Jan. 01, 2017
Presidio Group, Inc. [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Jan. 01, 2017
Commercial Insurance Brokers [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Apr. 01, 2017
Williams - Manny Insurance Group [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | May 01, 2017
GPL Assurance Inc. (GPL) [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Aug. 01, 2017
Lutgert Insurance (LI) [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Sep. 01, 2017
DiBrina Group (DBG) [Member] |
Business Acquisition [Line Items] |
Effective Date of Acquisition | Oct. 01, 2017
31 Other Acquisitions [Member] |
Business Acquisition [Line Items] |
Effective Year of Acquisition | 2017

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