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Organization and Basis of Presentation Revenue Recognition (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]      
Deferred revenue, current $ 34.1 $ 24.6  
Deferred revenue, noncurrent 78.8 70.8  
Deferred revenue, revenue recognized 26.8 24.9 $ 23.4
Increase Installed Capacity [Member]      
Deferred Revenue Arrangement [Line Items]      
Deferred revenue, additions 5.0 $ 20.0  
Deferred revenue, current 3.7    
Deferred revenue, noncurrent $ 21.3