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Revenue Recognition - Graduation Fund (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Graduation Fund [Roll Forward]    
Balance at beginning of period $ 43,329 $ 37,400
Revenue deferred 30,071 27,349
Benefit redeemed (23,759) (21,420)
Balance at end of period $ 49,641 $ 43,329