Summary of Significant Accounting Policies - Revenue by Vertical Market as Percentage of Total Revenue (Details) - Customer Concentration Risk [Member] - Sales Revenue, Services, Net [Member]	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 30, 2017	Jun. 29, 2018	Jun. 30, 2017
Concentration Risk [Line Items]
Total revenue	100.00%	100.00%	100.00%	100.00%
Technology Service Providers [Member]
Concentration Risk [Line Items]
Total revenue	32.50%	32.60%	32.70%	33.10%
Manufacturing [Member]
Concentration Risk [Line Items]
Total revenue	18.40%	26.00%	18.90%	25.80%
Healthcare [Member]
Concentration Risk [Line Items]
Total revenue	16.00%	17.40%	15.60%	17.10%
Financial Services [Member]
Concentration Risk [Line Items]
Total revenue	15.70%	8.10%	15.10%	8.00%
Energy [Member]
Concentration Risk [Line Items]
Total revenue	5.00%	4.70%	4.90%	4.90%
General Markets [Member]
Concentration Risk [Line Items]
Total revenue	12.40%	11.20%	12.80%	11.10%