Stock-Based Compensation Included in Operating Expense Line-Items (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 5,369	$ 4,024	$ 3,818
Instructional costs and services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	1,274	876	896
Selling and promotional
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	568	444	378
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense in operating income	$ 3,527	$ 2,704	$ 2,544