Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Total revenue	$ 239,790	$ 224,249	$ 745,286	$ 722,980
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	153,794	146,145	493,311	478,485
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	76,960	70,069	225,342	220,152
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	9,036	8,035	26,633	24,343
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	153,794	146,145	493,311	478,485
Accounting, Tax, Advisory and Consulting [Member] | Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	153,794	146,145	493,311	478,485
Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	73,409	67,192	216,394	210,292
Core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	73,409	67,192	216,394	210,292
Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,551	2,877	8,948	9,860
Non-core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	3,551	2,877	8,948	9,860
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,553	5,902	19,499	18,211
Managed Networking, Hardware Services [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,553	5,902	19,499	18,211
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,483	2,133	7,134	6,132
National Practices Consulting [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 2,483	$ 2,133	$ 7,134	$ 6,132