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Deferred revenue (Tables)
12 Months Ended
Dec. 31, 2018
Subclassifications of assets, liabilities and equities [abstract]  
Explanation of significant changes in contract liabilities
Contract assets
December 31, 2018

At January 1, 2018
$
1,004

Additions to contract assets
9,030

Invoiced during the year
(940
)
At December 31, 2018
$
9,094

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