Acquisitions - Additional Information (Detail) $ in Thousands	6 Months Ended		9 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Client_List shares	Sep. 30, 2017 USD ($) Client_List shares	Dec. 31, 2017 USD ($)
Business Acquisition Contingent Consideration [Line Items]
Goodwill			$ 560,712		$ 528,424
Contingent consideration	$ 12,400	$ 17,500	12,400	$ 17,500
Fair value of contingent consideration			3,300	(200)
Consideration paid in cash			$ 11,000	$ 4,800
Consideration paid in cash	$ 23,400	$ 23,700
Number of common stock issued | shares			100,000	200,000
CMF Associates, LLC (“CMF”), Slaton Insurance (“Slaton”) and Pacific Coastal Pension and Insurance Services, Inc. (Pacific Coastal”) [Member]
Business Acquisition Contingent Consideration [Line Items]
Goodwill			$ 32,255	$ 39,460
Acquisition of Client Lists [Member]
Business Acquisition Contingent Consideration [Line Items]
Number of client list purchased | Client_List				2
Acquisition of Client Lists [Member] | Benefits and Insurance Services Practice Group [Member]
Business Acquisition Contingent Consideration [Line Items]
Number of client list purchased | Client_List				1
Contingent consideration				$ 700
Acquisition of Client Lists [Member] | Financial Services Practice Group [Member]
Business Acquisition Contingent Consideration [Line Items]
Number of client list purchased | Client_List			1	1
Contingent consideration			$ 300	$ 700
Previous Client List Purchases [Member]
Business Acquisition Contingent Consideration [Line Items]
Consideration paid in cash			$ 1,100	$ 1,000