Revenue - Changes in contract assets and contract liabilities (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Contract assets
Transferred to receivables from contract assets recognized at the beginning of the period	$ (5,502)
Revenue recognized and not billed as of the end of the period	2,432
Net change in contract assets	(3,070)
Contract liabilities
Revenue recognized that was included in the contract liability balance as the beginning of the period	4,608
Increases due to cash received, net of amounts recognized as revenue during the period	(6,919)
Net change in contract liabilities	$ (2,311)