Revenue - Changes in contract assets and contract liabilities (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Contract assets
Transferred to receivables from contract assets recognized at the beginning of the period	$ (137)
Revenue recognized and not billed as of the end of the period	1,181
Net change in contract assets	1,044
Contract liabilities
Revenue recognized that was included in the contract liability balance as of the beginning of the period	4,456
Increases due to cash received, net of amounts recognized as revenue during the period	(1,792)
Net change in contract liabilities	$ 2,664