Revenue Recognition - Graduation Fund (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)
Number of free courses | item	1
Number of successfully completed courses | item	3
Consecutive terms of non attendance in which Graduation Fund credits will be lost | item	1
Expected collection period of tuition receivable, current	12 months
Graduation fund estimated to be redeemed	$ 20,300
Balance at beginning of year	37,400	$ 29,499
Revenue deferred	18,791	17,494
Benefit redeemed	(15,792)	(12,551)
Balance at end of year	40,399	$ 34,442
Unbilled receivables	$ 2,000
Minimum
Expected collection period of tuition receivable, noncurrent	2 years
Maximum
Expected collection period of tuition receivable, noncurrent	4 years