BALANCE SHEET COMPONENTS Balance Sheet Components (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 145,095	$ 135,311
Restricted cash	3,279	3,167
Cash	148,374	138,478	$ 137,664	$ 128,884
Inventory, Finished Goods, Gross	7,524	7,779
Inventory, Raw Materials and Supplies, Gross	572	799
Inventory Valuation Reserves	(152)	(180)
Inventory, Net	7,944	8,398
Accrued Sales Returns	689	614
Accrued Employee Benefits, Current	5,316	5,568
Accrued Warehousing And Inventory Related Expense	2,544	1,423
Other Accrued Liabilities, Current	8,549	7,605
Deferred Revenue, Current	4,112	4,455
Cash Equivalents [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	74,846	73,608
Cash [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and Cash Equivalents, at Carrying Value	70,249	61,703
Enrollment Package Revenue [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Deferred Revenue	355	379
Auto Ship Advances [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Deferred Revenue	1,740	1,665
Unshipped Product [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Deferred Revenue	$ 2,017	$ 2,411