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. 29, 2017	Sep. 30, 2016	Sep. 29, 2017	Sep. 30, 2016
Concentration Risk [Line Items]
Total revenue	100.00%	100.00%	100.00%	100.00%
Technology Service Providers [Member]
Concentration Risk [Line Items]
Total revenue	33.30%	36.40%	33.10%	35.20%
Manufacturing [Member]
Concentration Risk [Line Items]
Total revenue	24.20%	24.60%	25.30%	24.10%
Healthcare [Member]
Concentration Risk [Line Items]
Total revenue	16.60%	17.10%	17.00%	18.50%
Financial Services [Member]
Concentration Risk [Line Items]
Total revenue	9.50%	7.70%	8.50%	7.60%
Energy [Member]
Concentration Risk [Line Items]
Total revenue	4.90%	4.90%	4.90%	5.40%
General Markets [Member]
Concentration Risk [Line Items]
Total revenue	11.50%	9.30%	11.20%	9.20%