Business Combinations - Acquisition Method for Recording Business Combinations (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2015
e3 Financial, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jan. 01, 2015
Aequus Trade Credit LLC [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jan. 31, 2015
Cohen & Lord Insurance Brokers Limited [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Feb. 01, 2015
Cohn Financial Group, LLC (CFG) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Feb. 01, 2015
Excel Insurance Services, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Feb. 01, 2015
Metcom Excess [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Feb. 01, 2015
NationAir Aviation Insurance [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Feb. 01, 2015
Evolution Group of Companies [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Feb. 06, 2015
Burns-Fazzi, Brock & Associates, LLC (BFB) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Apr. 01, 2015
Madison Risk & Insurance Services, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Apr. 01, 2015
Integrated Healthcare Strategies, LLC (IHS) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	May 01, 2015
James R. Weir Insurance Agency, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	May 01, 2015
McDowall Associates Human Resources Consultants Ltd. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	May 01, 2015
Vital Benefits, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	May 01, 2015
Monument, LLC (ML) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jun. 01, 2015
Solid Benefit Guidance Limited Liability Company (SBG) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jun. 01, 2015
ARM Re Ltda Corredores de Reaseguros [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Jul. 01, 2015
National Administration Company, Inc. (NAC) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Aug. 01, 2015
William Gallagher Associates Insurance Brokers, Inc. (WGA) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Aug. 01, 2015
North Alabama Insurance, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Sep. 01, 2015
Burkwald & Associates, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Oct. 01, 2015
Sigma II Insurance Agency [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Oct. 01, 2015
Christie Phoenix (Victoria) Ltd. and Discovery Insurance Services Ltd [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Nov. 01, 2015
Reid Manson Limited [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Nov. 01, 2015
Centennial Insurance Agency, LLC [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Dec. 01, 2015
Hawk Agency, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Dec. 01, 2015
Managed Healthcare Solutions, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Dec. 01, 2015
McPherson Benefits Group, Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Dec. 01, 2015
Strathearn Insurance Group Pty.Ltd (SIG) [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Dec. 01, 2015
Fifteen Other Acquisitions [Member]
Business Acquisition [Line Items]
Effective Year of Acquisition	2015