Revenue Net contract assets (liabilities) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Net contract assets (liabilities) [Abstract]
Contract with Customer, Asset, Gross, Current	$ 3,997	$ 14,757
Contract with Customer, Asset, Explanation of Change	(10,760)
Contract with Customer, Liability, Current	(55)	(143)
Contract with Customer, Liability, Explanation of Change	88
Contract with Customer, Asset, Net, Current	3,942	$ 14,614
Contract with Customer, Net, Explanation of Change	$ (10,672)