Acquisitions - Additional Information (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) Client_List shares	Jun. 30, 2017 USD ($) Client_List shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Business Acquisition Contingent Consideration [Line Items]
Goodwill	$ 560,688			$ 528,424
Contingent consideration	12,400	$ 17,500
Fair value of contingent consideration	3,100	800
Consideration paid in cash	4,100	4,800
Consideration paid in cash	$ 23,400	$ 23,700
Number of common stock issued | shares	56,000	177,000
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	$ 32,300		$ 39,500
Acquisition of Client Lists [Member]
Business Acquisition Contingent Consideration [Line Items]
Contingent consideration		$ 700
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	0	1
Previous Client List Purchases [Member]
Business Acquisition Contingent Consideration [Line Items]
Consideration paid in cash	$ 600	$ 700