Acquisitions - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($) Business Client_List shares	Sep. 30, 2015 USD ($) Client_List shares	Dec. 31, 2015 USD ($)
Business Acquisition, Contingent Consideration [Line Items]
Number of businesses acquired | Business	4
Consideration paid in cash		$ 300
Contingent consideration	$ 17,800	3,000
Goodwill	478,551		$ 447,685
Guaranteed future consideration		800
Fair value of contingent consideration	900	3,100
Consideration paid in cash	$ 5,700	$ 7,800
Number of common stock issued | shares	220,000	215,000
Contingent consideration, current	$ 16,166		12,855
Contingent consideration, non-current	17,748		$ 11,962
Employee Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client_List		5
Contingent Purchase Price Liability Current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	5,300
Contingent Purchase Price Liability Non-current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	12,500
Millimaki Eggert, L.L.P., (“Millimaki”), The Savitz Organization, (“Savitz”), Flex-Pay Business Services, Inc., (“Flex-Pay”) and Ed Jacobs & Associates, Inc. (“EJ&A”) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	36,700
Consideration paid in common stock	1,600
Contingent consideration	17,800
Goodwill	30,900
Millimaki Eggert, L.L.P., (“Millimaki”), The Savitz Organization, (“Savitz”), Flex-Pay Business Services, Inc., (“Flex-Pay”) and Ed Jacobs & Associates, Inc. (“EJ&A”) [Member] | Employee Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	29,900
Millimaki Eggert, L.L.P., (“Millimaki”), The Savitz Organization, (“Savitz”), Flex-Pay Business Services, Inc., (“Flex-Pay”) and Ed Jacobs & Associates, Inc. (“EJ&A”) [Member] | Financial Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	1,000
Millimaki Eggert, L.L.P., ("Millimaki"), The Savitz Organization, ("Savitz"), Flex-Pay Business Services, Inc., ("Flex-Pay") and Ed Jacobs & Associates, Inc. (“EJ&A”) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	30,927
Millimaki Eggert, L.L.P., ("Millimaki"), The Savitz Organization, ("Savitz"), Flex-Pay Business Services, Inc., ("Flex-Pay") and Ed Jacobs & Associates, Inc. (“EJ&A”) [Member] | Employee Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	30,900
Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	800
Contingent consideration	$ 700
Number of client list purchased | Client_List	6
Guaranteed future consideration	$ 1,200
Acquisition of Client Lists [Member] | Employee Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client_List	5
Acquisition of Client Lists [Member] | Financial Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | Client_List	1
Model Consulting, Inc [Member]
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	$ 5,500
Contingent consideration	4,200	$ 4,200
Goodwill	$ 6,900	6,865
Contingent consideration, current		1,600
Contingent consideration, non-current		2,600
Model Consulting, Inc [Member] | Employee Services [Member]
Business Acquisition, Contingent Consideration [Line Items]
Goodwill		$ 6,900