Stock Options, Restricted Stock and Restricted Stock Units - Stock-based compensation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 2,036,000	$ 2,472,000	$ 4,926,000	$ 4,923,000
Tax benefit	802,000	986,000	1,900,000	1,942,000
Stock-based compensation expense, net of income tax	1,234,000	1,486,000	3,026,000	2,981,000
Tax shortfall related to share-based payment arrangements			$ 50,000	$ 25,000
Stock options exercised			0	0
Instruction and educational support [Member]
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	(438,000)	546,000	$ 155,000	$ 914,000
General and administration [Member]
Schedule of stock-based compensation expense
Stock-based compensation expense included in operating expense	$ 2,474,000	$ 1,926,000	$ 4,771,000	$ 4,009,000