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Revenue Recognition - Graduation Fund (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
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 12 months  
Balance at beginning of year $ 37,400 $ 29,499
Revenue deferred 27,349 25,360
Benefit redeemed (21,420) (17,459)
Balance at end of year 43,329 $ 37,400
Unbilled receivables 1,100  
Current Liability [Member]    
Graduation fund estimated to be redeemed $ 20,800  
Minimum    
Expected collection period of tuition receivable, noncurrent 2 years  
Maximum    
Expected collection period of tuition receivable, noncurrent 4 years