Valuation and Qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation Allowances and Reserves, Opening Balance	$ 31	$ 57	$ 353
Valuation Allowances and Reserves, Charged to Cost and Expense	5	(19)	(46)
Valuation Allowances and Reserves, Adjustments	0	7	250
Valuation Allowances and Reserves, Closing Balance	36	31	57
Allowance for Sales Returns [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation Allowances and Reserves, Opening Balance	490	401	530
Valuation Allowances and Reserves, Charged to Cost and Expense	3,713	3,442	3,729
Valuation Allowances and Reserves, Adjustments	3,675	3,353	3,858
Valuation Allowances and Reserves, Closing Balance	$ 528	$ 490	$ 401