Summary of Significant Accounting Policies - Revenue by Vertical Market as Percentage of Total Revenue (Details) - Customer Concentration Risk [Member] - Sales Revenue, Services, Net [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Total revenue	100.00%	100.00%	100.00%
Technology Service Providers [Member]
Concentration Risk [Line Items]
Total revenue	32.40%	33.10%	35.20%
Manufacturing [Member]
Concentration Risk [Line Items]
Total revenue	19.50%	24.30%	24.20%
Financial Services [Member]
Concentration Risk [Line Items]
Total revenue	15.20%	9.10%	7.80%
Energy [Member]
Concentration Risk [Line Items]
Total revenue	4.70%	5.00%	5.30%
General Markets [Member]
Concentration Risk [Line Items]
Total revenue	12.00%	11.70%	9.30%
Healthcare [Member]
Concentration Risk [Line Items]
Total revenue	16.20%	16.80%	18.20%