| Schedule of Additional Balance Sheet Components [Table Text Block] | 
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   | 
December 31,  | 
 
   | 
2017  | 
   | 
2016  | 
 
Cash and cash equivalents:  | 
   | 
   | 
   | 
 
Cash  | 
$  | 
61,703  | 
 
  | 
   | 
$  | 
52,453  | 
 
  | 
 
Cash equivalents  | 
73,608  | 
 
  | 
   | 
73,468  | 
 
  | 
 
   | 
$  | 
135,311  | 
 
  | 
   | 
$  | 
125,921  | 
 
  | 
 
Inventories:  | 
   | 
   | 
   | 
 
Finished goods  | 
$  | 
7,779  | 
 
  | 
   | 
$  | 
10,722  | 
 
  | 
 
Raw materials  | 
799  | 
 
  | 
   | 
617  | 
 
  | 
 
Inventory reserve for obsolescence  | 
(180  | 
)  | 
   | 
(82  | 
)  | 
 
   | 
$  | 
8,398  | 
 
  | 
   | 
$  | 
11,257  | 
 
  | 
 
Property and equipment:  | 
   | 
   | 
   | 
 
Office equipment  | 
$  | 
530  | 
 
  | 
   | 
$  | 
517  | 
 
  | 
 
Office software  | 
916  | 
 
  | 
   | 
672  | 
 
  | 
 
Machinery  | 
30  | 
 
  | 
   | 
28  | 
 
  | 
 
Furniture and fixtures  | 
276  | 
 
  | 
   | 
241  | 
 
  | 
 
Leasehold improvements  | 
957  | 
 
  | 
   | 
840  | 
 
  | 
 
Construction in progress (including internal-use software development costs)  | 
10  | 
 
  | 
   | 
157  | 
 
  | 
 
Property and equipment, at cost  | 
2,719  | 
 
  | 
   | 
2,455  | 
 
  | 
 
Accumulated depreciation and amortization  | 
(1,570  | 
)  | 
   | 
(1,067  | 
)  | 
 
   | 
$  | 
1,149  | 
 
  | 
   | 
$  | 
1,388  | 
 
  | 
 
Other accrued expenses:  | 
   | 
   | 
   | 
 
Sales returns  | 
$  | 
614  | 
 
  | 
   | 
$  | 
1,632  | 
 
  | 
 
Employee-related expense  | 
5,568  | 
 
  | 
   | 
10,541  | 
 
  | 
 
Warehousing, inventory-related and other  | 
1,423  | 
 
  | 
   | 
2,816  | 
 
  | 
 
   | 
$  | 
7,605  | 
 
  | 
   | 
$  | 
14,989  | 
 
  | 
 
Deferred revenue:  | 
   | 
 
  | 
   | 
   | 
 
  | 
 
Unshipped product  | 
$  | 
2,411  | 
 
  | 
   | 
$  | 
2,191  | 
 
  | 
 
Auto ship advances  | 
1,665  | 
 
  | 
   | 
2,327  | 
 
  | 
 
Enrollment package revenue  | 
379  | 
 
  | 
   | 
430  | 
 
  | 
 
   | 
$  | 
4,455  | 
 
  | 
   | 
$  | 
4,948  | 
 
  | 
 
  
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