Business Combinations - Additional Information (Details) shares in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) business clientList shares	Dec. 31, 2019 USD ($) business clientList shares	Dec. 31, 2018 USD ($)
Business Acquisition, Contingent Consideration [Line Items]
Number of businesses acquired | business	7	6
Consideration paid in cash	$ 73,400,000	$ 19,400,000
Funds held for clients	1,700,000	6,900,000
Cash	500,000	800,000
Consideration paid in common stock	8,700,000	2,000,000.0
Contingent consideration	38,900,000	11,200,000
Contingent consideration arrangements - High	43,000,000.0	11,500,000
Aggregate fair value of contingent consideration	38,000,000.0	10,300,000
Contingent consideration, current	20,288,000	16,193,000
Contingent consideration, non-current	34,103,000	15,896,000
Annual aggregate revenue	45,200,000	17,400,000
Goodwill	683,083,000	588,213,000	$ 564,300,000
Decrease in fair value of contingent consideration	600,000	1,600,000
Consideration paid in cash	$ 12,500,000	$ 16,900,000
Number of common stock issued (in shares) | shares	0.1	0.1
Benefits & Insurance
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	$ 313,378,000	$ 252,300,000	233,008,000
Financial Services
Business Acquisition, Contingent Consideration [Line Items]
Goodwill	368,039,000	334,247,000	$ 329,626,000
Business Acquisitions in 2020 and 2019
Business Acquisition, Contingent Consideration [Line Items]
Funds held for clients	1,716,000	6,878,000
Cash	536,000	826,000
Goodwill	97,439,000	24,369,000
Acquisition of Client Lists
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	600,000	300,000
Contingent consideration	$ 400,000	$ 200,000
Acquisition of Client Lists | Benefits & Insurance
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | clientList	3	1
Acquisition of Client Lists | Financial Services
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased | clientList	1
Previous Client List Purchases
Business Acquisition, Contingent Consideration [Line Items]
Consideration paid in cash	$ 400,000	$ 900,000
Contingent Purchase Price Liability – Current
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration, current	10,900,000	2,800,000
Contingent Purchase Price Liability – Non-current
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration, non-current	$ 27,100,000	$ 7,500,000