Segment Disclosures - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 282,719	$ 238,389	$ 862,097	$ 752,787
Financial Services
Disaggregation of Revenue [Line Items]
Total revenue	187,232	155,499	577,970	498,359
Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue	85,797	73,881	255,656	227,433
National Practices
Disaggregation of Revenue [Line Items]
Total revenue	9,690	9,009	28,471	26,995
Accounting, tax, advisory and consulting
Disaggregation of Revenue [Line Items]
Total revenue	187,232	155,499	577,970	498,359
Accounting, tax, advisory and consulting | Financial Services
Disaggregation of Revenue [Line Items]
Total revenue	187,232	155,499	577,970	498,359
Accounting, tax, advisory and consulting | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Accounting, tax, advisory and consulting | National Practices
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Core benefits and insurance services
Disaggregation of Revenue [Line Items]
Total revenue	83,009	71,497	246,367	219,362
Core benefits and insurance services | Financial Services
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Core benefits and insurance services | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue	83,009	71,497	246,367	219,362
Core benefits and insurance services | National Practices
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Non-core benefits and insurance services
Disaggregation of Revenue [Line Items]
Total revenue	2,788	2,384	9,289	8,071
Non-core benefits and insurance services | Financial Services
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Non-core benefits and insurance services | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue	2,788	2,384	9,289	8,071
Non-core benefits and insurance services | National Practices
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Managed networking, hardware services
Disaggregation of Revenue [Line Items]
Total revenue	7,088	6,592	20,952	19,748
Managed networking, hardware services | Financial Services
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Managed networking, hardware services | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
Managed networking, hardware services | National Practices
Disaggregation of Revenue [Line Items]
Total revenue	7,088	6,592	20,952	19,748
National practices consulting
Disaggregation of Revenue [Line Items]
Total revenue	2,602	2,417	7,519	7,247
National practices consulting | Financial Services
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
National practices consulting | Benefits & Insurance
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0	0
National practices consulting | National Practices
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,602	$ 2,417	$ 7,519	$ 7,247