Revenue Recognition - Graduation Fund (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Graduation Fund [Roll Forward]
Balance at beginning of period	$ 49,641	$ 43,329
Revenue deferred	13,942	14,787
Benefit redeemed	(11,235)	(12,025)
Balance at end of period	$ 52,348	$ 46,091