Revenue Recognition - contract assets and liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Dec. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2015
Contract assets.
Contract assets	$ 1,000	$ 1,000	$ 1,000	$ 1,000
Contract liabilities:
Deferred Revenue, Beginning Balance		400	92
Additions			470
Deductions		(47)	(53)
Deferred Revenue, Ending Balance	$ 400	$ 353	$ 509