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		9 Months Ended
	Sep. 25, 2020	Sep. 27, 2019	Sep. 25, 2020	Sep. 27, 2019
Concentration Risk [Line Items]
Total revenue	100.00%	100.00%	100.00%	100.00%
Technology Service Providers [Member]
Concentration Risk [Line Items]
Total revenue	33.10%	32.90%	33.30%	32.40%
Manufacturing [Member]
Concentration Risk [Line Items]
Total revenue	13.70%	17.60%	14.10%	17.20%
Financial Services [Member]
Concentration Risk [Line Items]
Total revenue	14.80%	13.00%	14.60%	13.60%
Energy [Member]
Concentration Risk [Line Items]
Total revenue	5.60%	5.40%	6.30%	5.20%
General Markets [Member]
Concentration Risk [Line Items]
Total revenue	18.40%	14.20%	17.90%	14.90%
Healthcare [Member]
Concentration Risk [Line Items]
Total revenue	14.40%	16.90%	13.80%	16.70%