Share-Based Compensation Expense (Schedule of Share-based Compensation Expense Showing Amount Attributable to Different Categories) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Total share-based compensation expense	$ 10,027	$ 8,792	$ 6,755
Cost of Services
Total share-based compensation expense	5,697	5,214	3,885
General and Administrative Expenses
Total share-based compensation expense	$ 4,330	$ 3,578	$ 2,870