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REVENUES - Revenues by Service Line (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of operating segments [line items]      
Revenues $ 1,430 $ 1,619 $ 1,561
Gas Transmission      
Disclosure of operating segments [line items]      
Revenues 942 1,141 1,112
Distribution      
Disclosure of operating segments [line items]      
Revenues 346 307 286
Connections      
Disclosure of operating segments [line items]      
Revenues 118 152 139
Other      
Disclosure of operating segments [line items]      
Revenues $ 24 $ 19 $ 24