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Revenue Recognition (Narrative) (Details) - USD ($)
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]            
Deferred Revenue $ 67,681,000 $ 63,500,000 $ 59,915,000 $ 52,620,000 $ 49,345,000 $ 45,760,000
Deferred Revenue, Current 28,795,000     22,265,000    
Deferred Revenue, Noncurrent $ 38,886,000     $ 30,355,000