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Acquisitions (Pro Forma Summary) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Business Acquisition [Line Items]        
Operating revenues     $ 921,121 $ 950,421
Net income     $ 211,921 $ 122,605
Peoples Gas Acquisition [Member]        
Business Acquisition [Line Items]        
Operating revenues $ 384,468 $ 351,758    
Net income $ 74,629 $ 54,530