Revenue Recognition - Changes in contract liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Contract liabilities:
Deferred revenue, Balance at Beginning of Period	$ 2,424	$ 2,421	$ 3,099	$ 2,932
Deferred revenue, Billings	2,514	1,991	4,514	4,176
Deferred revenue, Revenue Recognized	(1,290)	(1,297)	(3,965)	(3,993)
Deferred revenue, Balance at End of Period	$ 3,648	$ 3,115	$ 3,648	$ 3,115