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Property and Equipment
 12 Months Ended
Dec. 31, 2017
Property, Plant and Equipment [Abstract]  
Property and Equipment
Property and Equipment:

A summary of property and equipment at December 31, 2016 and 2017 is as follows:

 
Useful Lives
 
December 31,
 
(years)
 
2016
 
2017
 
 
 
 
 
 
Leasehold improvements
Shorter of lease term or economic life
 
$
4,911

 
$
16,039

Website and software development costs
1-3
 
10,114

 
14,501

Computer and office equipment
3
 
2,846

 
2,192

Software
3
 
1,367

 
685

Furniture
7
 
2,406

 
3,039

Construction in progress
 
 
10,856

 
—

 
 
 
32,500

 
36,456

Accumulated depreciation and amortization
 
 
(13,274
)
 
(14,138
)
Property and equipment, net
 
 
$
19,226

 
$
22,318




Depreciation and amortization expense for property and equipment amounted to $3,907, $5,805, and $6,688 for the years ended December 31, 2015, 2016, and 2017, respectively.