Revenue - Changes in contract assets and contract liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Contract assets
Transferred to receivables from contract assets recognized at the beginning of the period	$ (1,252)
Revenue recognized and not billed as of the end of the period	11,175
Net change in contract assets	9,923
Contract liabilities
Revenue recognized that was included in the contract liability balance as of the beginning of the period	(10,172)
Increases due to cash received, net of amounts recognized as revenue during the period	13,175
Net change in contract liabilities	$ 3,003