Schedule II - Valuation And Qualifying Accounts (Details) - Allowance for sales returns - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation And Qualifying Accounts [Abstract]
Balance at beginning of period	$ 3,908	$ 660	$ 10,018
Additions charged to costs and expenses	48,098	33,797	85,429
Deductions	(48,100)	(30,549)	(94,787)
Balance at end of period	$ 3,906	$ 3,908	$ 660