Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Revenue	$ 922,003	$ 855,340	$ 799,832
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	600,926
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	288,437
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	32,640
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	600,926
Accounting, Tax, Advisory and Consulting [Member] | Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	600,926
Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	276,496
Core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	276,496
Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	11,941
Non-core Benefits and Insurance Services [Member] | Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	11,941
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	24,404
Managed Networking, Hardware Services [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	24,404
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	8,236
National Practices Consulting [Member] | National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	$ 8,236