Revenue Recognition (Narrative) (Details) - USD ($)	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Deferred Revenue	$ 72,391,000	$ 67,681,000	$ 59,915,000	$ 55,586,000	$ 52,620,000	$ 45,760,000
Deferred Revenue, Current	30,703,000			23,893,000
Deferred Revenue, Noncurrent	$ 41,688,000			$ 31,693,000