Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Bad Debts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 2,031	$ 1,336	$ 1,239
Charged to costs and expenses	1,031	983	493
Deductions	(925)	(288)	(396)
Balance at end of period	2,137	2,031	1,336
Sales Returns and Allowance [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	1,817	1,814	1,636
Charged to costs and expenses	424	268	373
Deductions	(22)	(265)	(195)
Balance at end of period	2,219	1,817	1,814
Deferred Tax Asset Valuation Allowance [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	441	124	293
Charged to costs and expenses	129	317	0
Deductions	0	0	(169)
Balance at end of period	$ 570	$ 441	$ 124