Revenue Recognition - Changes in contract liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 2,837	$ 3,099
Deferred revenue, Billings	1,148	772
Deferred revenue, Revenue Recognized	(1,368)	(1,355)
Deferred revenue, Balance at End of Period	$ 2,617	$ 2,516