Acquisitions - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Funds held for clients	$ 161,289	$ 203,112
Goodwill	564,300	528,424	$ 487,484
Laurus Transaction Advisors, LLC. (“Laurus”), InR Advisory Services, LLC (“InR”) and Sequoia Financial Group, LLC. (“Sequoia”) [Member]
Business Acquisition [Line Items]
Cash	306
Accounts receivable, net	1,958
Other assets	12
Identifiable intangible assets	5,539
Accrued liabilities	(1,753)
Total identifiable net assets	6,062
Goodwill	36,054
Aggregate purchase price	$ 42,116
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