Revenue Recognition Contract with customer, asset and liability (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Revenue Recognition [Abstract]
Liability recognized in revenue	$ 568
Contract liabilities - current	3,989	$ 2,072
Contract liabilities - noncurrent	2,042	2,020
Net contract liabilities	$ 6,031	$ 4,092