Significant Accounting Policies - Graduation Fund (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
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	12 months
Balance at beginning of year	$ 29,499,000	$ 20,937,000
Revenue deferred	25,360,000	20,766,000
Benefit Redeemed	(17,459,000)	(12,204,000)
Balance at end of year	37,400,000	29,499,000
Current Liability [Member]
Graduation fund estimated to be redeemed	19,100,000
Other Assets [Member]
Restricted cash
Minimum protective endowment	500,000
Other Current Assets [Member]
Restricted cash
Restricted cash	$ 15,000	$ 13,000