Revenue Recognition - Graduation Fund (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Graduation Fund [Roll Forward]
Balance at beginning of period	$ 53,314	$ 49,641
Revenue deferred	11,435	13,942
Benefit redeemed	(11,185)	(11,235)
Balance at end of period	$ 53,564	$ 52,348