Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 211,110	$ 238,389	$ 236,943	$ 277,455	$ 203,138	$ 239,790	$ 235,498	$ 269,998	$ 963,897	$ 948,424	$ 922,003
Financial Services
Disaggregation of Revenue [Line Items]
Total revenue									629,778	616,567	600,926
Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue									297,758	296,228	288,437
National Practices
Disaggregation of Revenue [Line Items]
Total revenue									36,361	35,629	32,640
Accounting, tax, advisory and consulting
Disaggregation of Revenue [Line Items]
Total revenue									629,778	616,567	600,926
Accounting, tax, advisory and consulting | Financial Services
Disaggregation of Revenue [Line Items]
Total revenue									629,778	616,567	600,926
Core Benefits and Insurance Services
Disaggregation of Revenue [Line Items]
Total revenue									286,361	283,783	276,496
Core Benefits and Insurance Services | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue									286,361	283,783	276,496
Non-core Benefits and Insurance Services
Disaggregation of Revenue [Line Items]
Total revenue									11,397	12,445	11,941
Non-core Benefits and Insurance Services | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue									11,397	12,445	11,941
Managed networking, hardware services
Disaggregation of Revenue [Line Items]
Total revenue									26,458	25,982	24,404
Managed networking, hardware services | National Practices
Disaggregation of Revenue [Line Items]
Total revenue									26,458	25,982	24,404
National Practices consulting
Disaggregation of Revenue [Line Items]
Total revenue									9,903	9,647	8,236
National Practices consulting | National Practices
Disaggregation of Revenue [Line Items]
Total revenue									$ 9,903	$ 9,647	$ 8,236