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Significant Accounting Policies - Graduation Fund (Details)
 12 Months Ended
Dec. 31, 2016
USD ($)
class
item
Dec. 31, 2015
USD ($)
Consecutive terms of non attendance in which Graduation Fund credits will be lost | item 1  
Number of free courses | class 1  
Number of successfully completed courses | class 3  
Balance at beginning of year $ 20,937,000 $ 9,706,000
Revenue deferred 20,766,000 16,219,000
Benefit redeemed (12,204,000) (4,988,000)
Balance at end of year 29,499,000 $ 20,937,000
Current Liability [Member]    
Graduation fund estimated to be redeemed 14,500,000  
Other Assets [Member]    
Restricted cash    
Minimum protective endowment 500,000  
Other Current Assets [Member]    
Restricted cash    
Restricted cash $ 13,000