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Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2017
Summary of Significant Accounting Policies [Abstract]  
Inventories
Inventories consist of the following (U.S. dollars in thousands):

  
December 31,   
 
  
2016
  
2017
 
       
Raw materials 
 
$
108,276
  
$
87,683
 
Finished goods 
  
141,660
   
165,771
 
  
$
249,936
  
$
253,454
 
Inventory Reserves
Reserves of inventories consist of the following (U.S. dollars in thousands):

  
2015
  
2016
  
2017
 
          
Beginning balance 
 
$
56,034
  
$
20,744
  
$
7,995
 
Additions 
  
38,605
   
24,906
   
16,382
 
Write-offs 
  
(73,895
)
  
(37,655
)
  
(16,296
)
Ending balance 
 
$
20,744
  
$
7,995
  
$
8,081
 
Estimated Useful Lives
Depreciation is recorded using the straight-line method over the following estimated useful lives:

 
Buildings
 
39 years
 
 
Furniture and fixtures
 
5 - 7 years
 
 
Computers and equipment
 
3 -- 5 years
 
 
Leasehold improvements
 
Shorter of estimated useful life or lease term
 
 
Scanners
 
3 years
 
 
Vehicles
 
3 - 5 years
 
Unrecognized Tax Benefits
A reconciliation of the beginning and ending amount of unrecognized tax benefits included in other liabilities is as follows (U.S. dollars in thousands):

  
2015
  
2016
  
2017
 
          
Gross balance at January 1 
 
$
5,987
  
$
7,772
  
$
5,290
 
Increases related to prior year tax positions
  
1,677
   
185
  
─
 
Decreases related to prior year tax positions
 
─
  
─
   
(277
)
Increases related to current year tax positions
  
1,119
   
918
   
669
 
Settlements 
 
─
   
(3,369
)
  
(159
)
Decreases due to lapse of statutes of limitations
  
(667
)
  
(252
)
  
(187
)
Currency adjustments 
  
(344
)
  
36
   
178
 
Gross balance at December 31 
 
$
7,772
  
$
5,290
  
$
5,514