Revenue Recognition - Graduation Fund (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Balance at beginning of period	$ 43,329	$ 37,400
Revenue deferred	6,945	5,885
Benefit redeemed	(5,798)	(4,967)
Balance at end of period	$ 44,476	$ 38,318