Regulatory Matters (Piedmont Narrative) (Details) - Piedmont - Tennessee Annual Review Mechanism - TPUC - USD ($) $ in Millions	Sep. 11, 2023	May 19, 2023	Oct. 10, 2022
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)	$ 29	$ 42
Approved return on equity (as a percent)			9.80%
Approved rate decrease amount by state commission	(40)
Reconciliation cost of service	$ 11