Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Total revenue	$ 235,498	$ 232,641	$ 505,496	$ 498,731
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	154,373	151,737	339,517	332,340
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	72,127	72,753	148,382	150,083
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	8,998	8,151	17,597	16,308
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	154,373	151,737	339,517	332,340
Accounting, Tax, Advisory and Consulting [Member] | Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	154,373	151,737	339,517	332,340
Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	69,447	68,978	142,985	143,100
Core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	69,447	68,978	142,985	143,100
Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,680	3,775	5,397	6,983
Non-core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,680	3,775	5,397	6,983
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,522	6,121	12,946	12,079
Managed Networking, Hardware Services [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,522	6,121	12,946	12,079
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,476	2,030	4,651	3,999
National Practices Consulting [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 2,476	$ 2,030	$ 4,651	3,999
Other National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue				230
Other National Practices [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue				$ 230