Fair Value Measurements (Quantitative Level 3 Fair Value Disclosures - Liabilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) $ / MMBTU	Dec. 31, 2017 USD ($) $ / MMBTU	Dec. 31, 2016 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Derivative Asset (Liability)	$ (113)	$ (114)	$ (166)
Piedmont Natural Gas [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Derivative Asset (Liability)	(141)	(142)	$ (187)
Natural Gas, derivative liability [Member] | Valuation Technique, Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Net Derivative Asset (Liability)	$ (141)	$ (142)
Natural Gas, derivative liability [Member] | Minimum [Member] | Valuation Technique, Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input Price Per Unit | $ / MMBTU	1.87	2.10
Natural Gas, derivative liability [Member] | Maximum [Member] | Valuation Technique, Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Input Price Per Unit | $ / MMBTU	2.95	2.88