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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