Revenue Recognition Contract with customer, asset and liability (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue Recognition [Abstract]
Liability recognized in revenue	$ 654	$ 1,222
Contract liabilities - current	3,847	3,847	$ 2,072
Contract liabilities - noncurrent	2,080	2,080	2,020
Net contract liabilities	$ 5,927	$ 5,927	$ 4,092