Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail)	12 Months Ended
	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