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. 28, 2019	Jun. 29, 2018	Jun. 28, 2019	Jun. 29, 2018
Concentration Risk [Line Items]
Total revenue	100.00%	100.00%	100.00%	100.00%
Technology Service Providers [Member]
Concentration Risk [Line Items]
Total revenue	31.70%	32.60%	32.10%	32.70%
Manufacturing [Member]
Concentration Risk [Line Items]
Total revenue	17.10%	18.40%	17.10%	18.90%
Financial Services [Member]
Concentration Risk [Line Items]
Total revenue	13.40%	15.70%	13.70%	15.10%
Energy [Member]
Concentration Risk [Line Items]
Total revenue	5.50%	5.00%	5.10%	4.90%
General Markets [Member]
Concentration Risk [Line Items]
Total revenue	15.60%	12.40%	15.50%	12.80%
Healthcare [Member]
Concentration Risk [Line Items]
Total revenue	16.70%	15.90%	16.50%	15.60%