Schedule II-Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 1,255	$ 1,255	$ 1,255
Additions	100	60	35
Deductions	(98)	(60)	(35)
Balance at end of year	1,257	1,255	1,255
Allowance For Sales Returns And Product Warranty [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	72,026	72,609	62,376
Additions	172,493	109,494	105,987
Deductions	(148,506)	(110,077)	(95,754)
Balance at end of year	96,013	72,026	72,609
Allowance For Price Protection [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	4,480	2,125	1,806
Additions	7,764	12,239	7,467
Deductions	(8,597)	(9,884)	(7,148)
Balance at end of year	$ 3,647	$ 4,480	$ 2,125