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Transactions with Major Customers and Affiliates (Tables)
12 Months Ended
Dec. 31, 2020
Transactions with Major Customers and Affiliates [Abstract]  
Schedule of revenue by major customers
Operating revenues received from three of our major customers in 2020, 2019 and 2018 are as follows (in millions): 
202020192018
Dominion Energy, Inc.$248.5 $250.7 $113.5 
Duke Energy Corporation232.8222.5194.5
The Southern Company, Inc.216.4 216.1 166.2