Revenue - Summary of Disaggregation of Revenue by Source (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Revenue	$ 232,641	$ 232,641	$ 211,016	$ 498,731	$ 452,475
Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue		151,737		332,340
Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		6,121		12,079
National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue		2,030		3,999
Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		68,978		143,100
Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		3,775		6,983
Other National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue				230
Financial Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		151,737		332,340
Financial Services [Member] | Accounting, Tax, Advisory and Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue		151,737		332,340
Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		72,753		150,083
Benefits and Insurance Services [Member] | Core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		68,978		143,100
Benefits and Insurance Services [Member] | Non-core Benefits and Insurance Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		3,775		6,983
National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue		8,151		16,308
National Practices [Member] | Managed Networking, Hardware Services [Member]
Disaggregation Of Revenue [Line Items]
Revenue		6,121		12,079
National Practices [Member] | National Practices Consulting [Member]
Disaggregation Of Revenue [Line Items]
Revenue		$ 2,030		3,999
National Practices [Member] | Other National Practices [Member]
Disaggregation Of Revenue [Line Items]
Revenue				$ 230