Acquisitions - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) Client_List shares	Mar. 31, 2018 USD ($) Client_List shares	Dec. 31, 2018 USD ($)
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	$ 566,009		$ 564,300
Changes in fair value of contingent consideration	(300)	$ 1,600
Consideration paid in cash	$ 2,700	$ 2,900
Number of common stock issued | shares	0	0
Wenner Group, LLC [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	$ 2,165
Laurus Transaction Advisors, LLC [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill		$ 22,943
Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client_List	0	0