Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation Of Revenue [Line Items]
Revenue	$ 266,090	$ 241,459
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	180,603
Core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Revenue	74,122
Non-core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Revenue	3,208
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	5,958
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	1,969
Other National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	230
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	180,603
Financial Services [Member] | Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	180,603
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	77,330
Benefits and Insurance Services [Member] | Core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Revenue	74,122
Benefits and Insurance Services [Member] | Non-core Benefits and Insurance [Member]
Disaggregation Of Revenue [Line Items]
Revenue	3,208
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	8,157
National Practices [Member] | Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	5,958
National Practices [Member] | National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	1,969
National Practices [Member] | Other National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	$ 230