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Deferred revenue
 12 Months Ended
Dec. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]  
Deferred revenue
Deferred revenue:
Deferred revenue (i.e. contract liabilities) represents cash received from customers in excess of revenue recognized on uncompleted contracts.

Deferred revenue
December 31, 2018

At January 1, 2018
$
8,082

Additions to deferred revenue
19,353

Revenue recognized during the year
(10,754
)
 
 
At December 31, 2018
$
16,681