Revenue Recognition - Graduation Fund (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)
Consecutive terms of non attendance in which Graduation Fund credits will be lost | item	1
Number of free courses | item	1
Number of successfully completed courses | item	3
Expected collection period of tuition receivable, current	12 months
Graduation fund estimated to be redeemed	$ 21,300
Unbilled receivables	1,200
Balance at beginning of year	37,400	$ 29,499
Revenue deferred	5,885	5,702
Benefit Redeemed	(4,967)	(3,563)
Balance at end of year	$ 38,318	$ 31,638
Minimum
Expected redemption period of Graduation Fund, non-current	2 years
Maximum
Expected redemption period of Graduation Fund, non-current	4 years