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Revenue Recognition - Graduation Fund (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Graduation Fund [Roll Forward]    
Balance at beginning of period $ 49,641 $ 43,329
Revenue deferred 19,044 21,358
Benefit redeemed (17,077) (17,947)
Balance at end of period $ 51,608 $ 46,740