Revenue Recognition Reconciliation of Activity Related to Allowance for Sales Returns (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Sales Return Reserve	$ 24,522	$ 15,470	$ 9,341	$ 8,148
Revenue Recognition, Sales Returns, Reserve for Sales Returns	52,088	37,521	38,444
Revenue Recognition, Sales Returns, Reserve for Sales Returns, Period Increase/(Decrease)	$ (43,036)	$ (31,392)	$ (37,251)