Revenue - Change in contract balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Contract assets:
Contract assets, beginning of period	$ 6,165
Decreases due to cash received	(3,016)
Changes to the contract assets at the beginning of the period as a result of changes in estimates	159
Increases, net of reclassifications to receivables	218
Contract assets, end of period	3,526
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	(2,618)	$ (1,160)
Increases due to cash received, excluding amounts recognized as revenue during the year	4,829
Contract liabilities, end of period	$ 7,141