Revenue Recognition - contract assets and liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016
Contract assets.
Contract assets	$ 1,000	$ 1,000	$ 1,000	$ 1,000
Contract liabilities:
Deferred Revenue, Beginning Balance	655		400	92
Additions				653
Deductions			(70)	(90)
Deferred Revenue, Ending Balance	$ 400	$ 92	$ 330	$ 655