Revenue Recognition - Graduation Fund (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Graduation Fund [Roll Forward]
Balance at beginning of period	$ 53,314	$ 49,641
Revenue deferred	6,516	7,179
Benefit redeemed	(5,421)	(5,608)
Balance at end of period	$ 54,409	$ 51,212