Regulatory Matters (Regulatory Matters Narrative - Piedmont) (Details) - Piedmont - USD ($) $ in Millions				1 Months Ended
	Mar. 22, 2021	Feb. 16, 2021	Jul. 02, 2020	Dec. 31, 2020
2020 Tennessee Rate Case | TPUC
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease), amount		$ 16	$ 30	$ 26
Increase in annual revenue, percent			15.00%
Requested return on equity, percent		9.80%
2019 North Carolina Rate Case, additional Agreement and Stipulation of Partial Settlement | 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%