Revenue - Change in contract balances (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Contract asset:
Contract asset, beginning of period	$ 1,842
Decreases due to cash received	(988)
Increases, excluding amounts transferred to receivables during the period	1,592
Contract asset, end of period	2,446
Contract liability
Contract liability, beginning of period	1,350
Revenue recognized that was included in the contract liability balance at the beginning of the period	(1,224)
Increases due to cash received, excluding amounts recognized as revenue during the period	1,756
Contract liability end of period	1,882
Contract liability, noncurrent	$ 0