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Significant Accounting Policies Deferred Revenue (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]        
Deferred Revenue $ 63,500,000 $ 49,345,000 $ 59,915,000 $ 45,760,000
New contracts sold 9,889,000 8,324,000    
Less: reductions for revenue recognized (6,304,000) (4,739,000)    
Deferred Revenue, Current 27,219,000 20,510,000    
Deferred Revenue, Noncurrent $ 36,281,000 $ 28,835,000