Acquisitions - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Business Acquisition [Line Items]
Cash	$ 800
Acquired client funds	6,900
Goodwill	588,213	$ 564,300
Business Acquisitions in 2019 and 2018 [Member]
Business Acquisition [Line Items]
Cash	826		$ 306
Accounts receivable, net	1,843		1,920
Acquired client funds	6,878
Operating lease right-of-use asset, net	2,789
Other assets	99		12
Identifiable intangible assets	7,725		3,864
Operating lease liability - current	(1,013)
Other current liability	(2,245)		(1,717)
Operating lease liability - noncurrent	(1,776)
Client fund obligations	(6,878)
Total identifiable net assets	8,248		4,385
Goodwill	24,369		32,255
Aggregate purchase price	$ 32,617		$ 36,640