Fair Value Measurements (Quantitative Level 3 Fair Value Disclosures - Liabilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / MMBTU	Dec. 31, 2016 USD ($) $ / MMBTU	Oct. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net Derivative Asset (Liability)	$ (114)	$ (166)		$ 10
Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net Derivative Asset (Liability)	(142)	(187)	$ (188)
Natural Gas, derivative liability [Member] | Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net Derivative Asset (Liability)	$ (142)	$ (187)
Natural Gas, derivative liability [Member] | Minimum [Member] | Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Input Price Per Unit | $ / MMBTU	2.10	2.31
Natural Gas, derivative liability [Member] | Maximum [Member] | Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Input Price Per Unit | $ / MMBTU	2.88	4.18