Acquisitions - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Business Client shares	Dec. 31, 2016 USD ($) Business Client_List shares	Dec. 31, 2015 USD ($) Business Client_List shares
Business Acquisition, Contingent Consideration [Line Items]
Number of businesses acquired | Business	4	6	3
Contingent consideration, current	$ 15,151	$ 16,322
Contingent consideration, non-current	22,423	17,387
Annual revenue	25,700	41,200	$ 12,100
Goodwill	528,424	487,484	447,685
Consideration paid in cash	$ 9,800	$ 7,100	$ 12,000
Number of common stock issued | shares	300,000	400,000	300,000
Changes in fair value of contingent consideration	$ 1,500	$ 1,300	$ 2,900
Pacific Coastal Pension and Insurance Services, Inc. (Pacific Coastal), CMF Associates, LLC (CMF), Slaton Insurance (Slaton) and McKay & Carnahan, Inc. (McKay) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	24,200
Consideration paid in common stock	2,000
Contingent consideration	19,300
Contingent arrangements arising from acquisitions	20,300
Fair value of obligation at acquisition date	19,300
Contingent consideration, current	6,300
Contingent consideration, non-current	13,000
Goodwill	40,587
Millimaki Eggert, LLP(Millimaki), The Savitz organization (Savitz), Flex-Pay Business Services, Inc (Flex-pay), Ed Jacobs & Associates, Inc (Ed Jacobs), Actuarial Consultants, Inc (ACT), The Seff Group,P.C (Seff) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash		40,000
Consideration paid in common stock		2,100
Contingent consideration		21,100
Contingent arrangements arising from acquisitions		23,500
Contingent consideration, current		6,600
Contingent consideration, non-current		14,500
Goodwill		34,803
Model Consulting, Inc. (Model),Pension Resource Group, Inc. (PRG),Cottonwood Group, Inc. (Cottonwood)[Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash			10,500
Consideration paid in common stock			1,400
Contingent consideration			8,500
Contingent arrangements arising from acquisitions			8,700
Contingent consideration, current			2,900
Contingent consideration, non-current			5,600
Goodwill			13,471
Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash		1,200	2,800
Contingent consideration	$ 1,400	$ 1,500	100
Number of client list purchased	2	7
Guaranteed future consideration		$ 1,200	$ 800
Acquisition of Client Lists [Member] | Financial Services Practice Group [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client_List		1
Acquisition of Client Lists [Member] | Maximum [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	$ 100
Acquisition of Client Lists [Member] | Financial Services Practice Group [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client	1
Acquisition of Client Lists [Member] | Benefit and Insurance Services Practice Group [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased	1	6	6