Summary of Significant Accounting Policies - Revenue by Vertical Market (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenue by vertical market as a percentage of total revenue
Total revenue	100.00%	100.00%	100.00%
Healthcare [Member]
Revenue by vertical market as a percentage of total revenue
Total revenue	33.00%	30.00%	27.00%
Technology Service Providers [Member]
Revenue by vertical market as a percentage of total revenue
Total revenue	31.00%	34.00%	36.00%
Financial Services [Member]
Revenue by vertical market as a percentage of total revenue
Total revenue	6.00%	7.00%	6.00%
Energy [Member]
Revenue by vertical market as a percentage of total revenue
Total revenue	6.00%	6.00%	7.00%
General Markets [Member]
Revenue by vertical market as a percentage of total revenue
Total revenue	24.00%	23.00%	24.00%