Acquisitions - Allocated Aggregate Purchase Price of Acquisitions (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Funds held for clients	$ 203,112	$ 213,457
Goodwill	528,424	487,484	$ 447,685
Pacific Coastal Pension and Insurance Services, Inc. (Pacific Coastal), CMF Associates, LLC (CMF), Slaton Insurance (Slaton) and McKay & Carnahan, Inc. (McKay) [Member]
Business Acquisition [Line Items]
Cash	843
Accounts receivable, net	4,338
Property and equipment	48
Other assets	221
Identifiable intangible assets	4,229
Accounts payable	(1,283)
Accrued liabilities	(3,503)
Total identifiable net assets	4,893
Goodwill	40,587
Aggregate purchase price	$ 45,480
Millimaki Eggert, LLP(Millimaki), The Savitz organization (Savitz), Flex-Pay Business Services, Inc (Flex-pay), Ed Jacobs & Associates, Inc (Ed Jacobs), Actuarial Consultants, Inc (ACT), The Seff Group,P.C (Seff) [Member]
Business Acquisition [Line Items]
Cash		10
Accounts receivable, net		6,649
Funds held for clients		37,230
Property and equipment		440
Other assets		294
Identifiable intangible assets		22,177
Accrued liabilities		(1,133)
Client fund obligations		(37,230)
Total identifiable net assets		28,437
Goodwill		34,803
Aggregate purchase price		$ 63,240
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