Schedule II - Valuation and Qualifying Accounts and Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 1,282	$ 1,315	$ 1,397
Additions Charged Against Revenue	0	0	0
Additions Charged to Other Accounts	453	648	276
Deductions	(286)	(681)	(358)
Balance at End of Year	1,449	1,282	1,315
Allowance for product returns
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	2,314	2,116	1,838
Additions Charged Against Revenue	2,055	2,071	1,559
Additions Charged to Other Accounts	0	0	0
Deductions	(1,898)	(1,873)	(1,281)
Balance at End of Year	$ 2,471	$ 2,314	$ 2,116