Revenue - Change in contract balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Contract assets:
Contract assets, beginning of period	$ 6,165
Decreases due to cash received	(4,147)
Changes to the contract assets at the beginning of the period as a result of changes in estimates	394
Increases, net of reclassifications to receivables	1,183
Contract assets, end of period	3,595
Contract liabilities:
Contract liabilities, beginning of period	4,930	$ 1,733
Revenue recognized that was included in the contract liabilities balance at the beginning of the period	(3,614)	$ (1,408)
Increases due to cash received, excluding amounts recognized as revenue during the year	5,403
Contract liabilities, end of period	$ 6,719