Revenue Recognition - Graduation Fund (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Graduation Fund [Roll Forward]
Balance at beginning of period	$ 49,641	$ 43,329
Revenue deferred	7,179	6,945
Benefit redeemed	(5,608)	(5,798)
Balance at end of period	$ 51,212	$ 44,476