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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Revenue Recognition [Abstract]    
Customer Accounts Receivable, Billed (Less Reserve) [1] $ 540.5 $ 477.0
Customer Accounts Receivable, Unbilled (Less Reserve) [2] 349.1 $ 378.6
Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) [1] 63.5  
Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) [2] $ (29.5)  
[1] Customer billed receivables increased over the period due to November 2018 being colder than November 2017, leading to more gas usage included in December bills.
[2] Customer unbilled receivables decreased over the period due December 2018 being warmer than December 2017, leading to less estimated gas usage.