Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Total revenue	$ 269,998	$ 266,090
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	185,144	180,603
Core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	73,538	74,122
Non-core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,717	3,208
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,424	5,958
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,175	1,969
Other [Member]
Disaggregation Of Revenue [Line Items]
Total revenue		230
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	185,144	180,603
Financial Services [Member] | Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	185,144	180,603
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	76,255	77,330
Benefits and Insurance Services [Member] | Core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	73,538	74,122
Benefits and Insurance Services [Member] | Non-core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	2,717	3,208
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	8,599	8,157
National Practices [Member] | Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	6,424	5,958
National Practices [Member] | National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Total revenue	$ 2,175	1,969
National Practices [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Total revenue		$ 230