Segment Information - The Company's Net Sales by Product and Service (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net sales by product and service:
Product sales	$ 187,591	$ 269,731	$ 253,041
Enrollment package revenue, freight and other	13,676	25,616	17,623
Less: sales returns	(3,704)	(7,619)	(5,804)
Total net sales	$ 197,563	$ 287,728	$ 264,860