Revenue Recognition - Graduation Fund (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Balance at beginning of period	$ 43,329	$ 37,400
Revenue deferred	14,787	12,917
Benefit redeemed	(12,025)	(10,497)
Balance at end of period	$ 46,091	$ 39,820