Acquisitions - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Business Client_List shares	Dec. 31, 2015 USD ($) Business Client_List shares	Dec. 31, 2014 USD ($) Business Client shares
Business Acquisition, Contingent Consideration [Line Items]
Number of businesses acquired | Business	6	3	6
Contingent consideration, current	$ 16,322	$ 12,855
Contingent consideration, non-current	17,387	11,962
Annual revenue	41,200
Goodwill	487,484	447,685	$ 435,231
Consideration paid in cash	$ 7,100	$ 12,000	$ 4,600
Number of common stock issued | shares	400,000	300,000	100,000
Fair value of contingent consideration	$ 1,300	$ 2,900	$ 3,900
Millimaki Eggert, LLP(Millimaki), The Savitz organization (Savitz), Flex-Pay Business Services, Inc (Flex-pay), Ed Jacobs & Associates, Inc (EJ&A), 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
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview), Lewis Birch & Ricardo, LLC (LBR), Tegrit Group (Tegrit), Rognstad’s Inc. d.b.a. Sattler Insurance Agency (Sattler) and Weekes & Callaway (W&C) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash			43,900
Consideration paid in common stock			2,900
Contingent consideration			19,400
Contingent arrangements arising from acquisitions			20,900
Goodwill			51,873
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview), Lewis Birch & Ricardo, LLC (LBR), Tegrit Group (Tegrit), Rognstad’s Inc. d.b.a. Sattler Insurance Agency (Sattler) and Weekes & Callaway (W&C) [Member] | Contingent Purchase Price Liability Current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration			5,000
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview), Lewis Birch & Ricardo, LLC (LBR), Tegrit Group (Tegrit), Rognstad’s Inc. d.b.a. Sattler Insurance Agency (Sattler) and Weekes & Callaway (W&C) [Member] | Contingent Purchase Price Liability Non-current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration			14,400
Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	1,200	2,800	1,000
Contingent consideration	$ 1,500	100	$ 200
Number of client list purchased	7		4
Guaranteed future consideration	$ 1,200	$ 800
Acquisition of Client Lists [Member] | Benefit and Insurance Services Practice Group [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased	6	6	1
Acquisition of Client Lists [Member] | Financial Services Practice Group [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client			3
Acquisition of Client Lists [Member] | Financial Services Practice Group [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client_List	1