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Revenues - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Disaggregation of Revenue [Line Items]        
Typical length of contract     Electricity and natural gas products may be sold as fixed-price or variable-price products. The typical length of a contract to provide electricity and/or natural gas is twelve months.  
Retail Revenues        
Disaggregation of Revenue [Line Items]        
Gross receipts taxes $ 0.4 $ 0.3 $ 0.7 $ 0.8
Retail Cost of Revenues        
Disaggregation of Revenue [Line Items]        
Gross receipts taxes $ 1.4 $ 1.8 $ 3.1 $ 4.5