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Variable Interest Entities (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Variable Interest Entity [Line Items]    
Investment in VIE $ 71 $ 69
T.E.S. Filer City (50%)    
Variable Interest Entity [Line Items]    
Ownership interest 50.00%  
Grayling (50%)    
Variable Interest Entity [Line Items]    
Ownership interest 50.00%  
Genesee (50%)    
Variable Interest Entity [Line Items]    
Ownership interest 50.00%  
Craven (50%)    
Variable Interest Entity [Line Items]    
Ownership interest 50.00%