Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 1,050	$ 1,049	$ 1,049
Other	0	0	0
Additions	50	99	60
Deductions	154	(98)	(60)
Balance at end of year	1,254	1,050	1,049
Allowance for sales returns
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	14,321	10,602	14,298
Other	(14,321)	0	0
Additions	0	26,419	19,892
Deductions	0	(22,700)	(23,588)
Balance at end of year	0	14,321	10,602
Allowance for price protection
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	3,245	4,185	2,107
Other	(3,245)	0	0
Additions	0	7,149	11,728
Deductions	0	(8,089)	(9,650)
Balance at end of year	$ 0	$ 3,245	$ 4,185