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Deferred Revenue (Tables)
12 Months Ended
Dec. 31, 2018
Deferred Revenue Disclosure [Abstract]  
Summary of Changes in Deferred Revenue

The following table presents the changes in deferred revenue for the periods presented (in thousands):

 

 

Year Ended December 31,

 

 

2018

 

 

2017

 

Balance at beginning of period

$

1,215

 

 

$

1,411

 

Deferral of revenue

 

587

 

 

 

1,476

 

Recognition of deferred revenue

 

(968

)

 

 

(1,672

)

Balance at end of period

$

834

 

 

$

1,215