Revenue Recognition - Changes in contract assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Contract assets:
Unbilled receivables, Balance at Beginning of Period	$ 3,659	$ 1,542	$ 3,279	$ 1,429
Unbilled receivables, Revenue Recognized In Advance of Billings	796	1,170	2,264	1,745
Unbilled receivables, Billings	(200)	(557)	(1,288)	(1,019)
Unbilled receivables, Balance at End of Period	$ 4,255	$ 2,155	$ 4,255	$ 2,155