Regulatory Matters (Regulatory Matters Narrative - Piedmont) (Details) - Piedmont - USD ($) $ in Millions					1 Months Ended
	Jul. 28, 2021	May 06, 2021	Mar. 22, 2021	Jul. 02, 2020	Dec. 31, 2020
2020 Tennessee Rate Case | TPUC
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount				$ 30	$ 26
Increase in annual revenue, percent				15.00%
Approved rate increase, amount		$ 16
Approved return on equity percentage		9.80%
2021 North Carolina Rate Case | NCUC
Public Utilities, General Disclosures [Line Items]
Increase in annual revenue, percent			10.00%
Requested rate increase (decrease), amended amount			$ 109
Plants rolled into base rate			30.00%
2021 North Carolina Rate Case | NCUC | Subsequent event
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amended amount	$ 97