Stock Based Compensation Expense Included in Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax	$ 11,506	$ 9,028	$ 6,745
Income tax benefit	(3,699)	(2,854)	(2,185)
Cost of Services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax	3,655	2,737	1,825
Operating Expenses
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation before tax	$ 7,851	$ 6,291	$ 4,920