Acquisitions - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 566,009	$ 564,300
Wenner Group, LLC [Member]
Business Acquisition [Line Items]
Accounts receivable, net	550
Identifiable intangible assets	654
Other current assets	5
Current liabilities	(288)
Total identifiable net assets	921
Goodwill	2,165
Aggregate purchase price	$ 3,086
Laurus Transaction Advisors, LLC [Member]
Business Acquisition [Line Items]
Accounts receivable, net			$ 1,255
Identifiable intangible assets			457
Current liabilities			(805)
Total identifiable net assets			907
Goodwill			22,943
Aggregate purchase price			$ 23,850