Segment Disclosures - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Total revenue	$ 236,943	$ 235,498	$ 514,398	$ 505,496
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	154,083	154,373	342,860	339,517
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	73,940	72,127	153,552	148,382
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	8,920	8,998	17,986	17,597
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	154,083	154,373	342,860	339,517
Accounting, Tax, Advisory and Consulting [Member] | Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	154,083	154,373	342,860	339,517
Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	71,393	69,447	147,865	142,985
Core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	71,393	69,447	147,865	142,985
Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,547	2,680	5,687	5,397
Non-core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,547	2,680	5,687	5,397
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,581	6,522	13,156	12,946
Managed Networking, Hardware Services [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,581	6,522	13,156	12,946
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,339	2,476	4,830	4,651
National Practices Consulting [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 2,339	$ 2,476	$ 4,830	$ 4,651