Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation Of Revenue [Line Items]
Revenue	$ 224,249	$ 207,723	$ 722,980	$ 660,198
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	146,145		478,485
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	5,902		18,211
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,133		6,132
Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	67,192		210,292
Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,877		9,860
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	146,145		478,485
Financial Services [Member] | Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	146,145		478,485
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	70,069		220,152
Benefits and Insurance Services [Member] | Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	67,192		210,292
Benefits and Insurance Services [Member] | Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,877		9,860
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue	8,035		24,343
National Practices [Member] | Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue	5,902		18,211
National Practices [Member] | National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue	$ 2,133		$ 6,132