Acquisitions - Allocated Aggregate Purchase Price of Acquisitions (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill	$ 447,685	$ 435,231	$ 384,697
Model Consulting, Inc. (Model),Pension Resource Group, Inc. (PRG),Cottonwood Group, Inc. (Cottonwood)[Member]
Business Acquisition [Line Items]
Accounts receivable, net	1,501
Other assets	52
Identifiable intangible assets	7,037
Accounts payable	(62)
Accrued liabilities	(1,552)
Total identifiable net assets	6,976
Goodwill	13,471
Aggregate purchase price	$ 20,447
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview),Tegrit Group(Tegrit), Sattler Insurance Agency (Sattler) and Lewis Birch & Richardo, LLC (LBR) [Member]
Business Acquisition [Line Items]
Cash		1,381
Accounts receivable, net		4,204
Other assets		464
Identifiable intangible assets		17,952
Accounts payable		(3,319)
Accrued liabilities		(3,513)
Income taxes payable		(1,058)
Deferred taxes		(1,834)
Total identifiable net assets		14,277
Goodwill		51,873
Aggregate purchase price		$ 66,150
Associated Insurance Agents (AIA), Minneapolis, Minnesota and Knight Field Fabry LLP (Knight) [Member]
Business Acquisition [Line Items]
Cash			202
Accounts receivable, net			578
Other assets			137
Identifiable intangible assets			3,002
Accounts payable			(835)
Accrued liabilities			(416)
Deferred taxes			(1,165)
Total identifiable net assets			1,503
Goodwill			9,278
Aggregate purchase price			$ 10,781