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Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail)
 9 Months Ended
Sep. 30, 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 - Mannny 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
Twenty- three Other Acquisitions [Member]  
Business Acquisition [Line Items]  
Effective Year of Acquisition 2017