Revenue Recognition - Changes in contract liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 2,617	$ 2,516	$ 2,837	$ 3,099
Deferred revenue, Billings	1,736	1,228	2,884	2,000
Deferred revenue, Revenue Recognized	(1,392)	(1,320)	(2,760)	(2,675)
Deferred revenue, Balance at End of Period	$ 2,961	$ 2,424	$ 2,961	$ 2,424