Fair Value Measurements (Quantitative Level 3 Fair Value Disclosure - Liabilities) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / MMBTU	Mar. 31, 2017 USD ($) $ / MMBTU	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net derivative asset (liability)	$ (124)	$ (135)	$ (114)	$ (166)
Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net derivative asset (liability)	(132)	$ (145)	(142)	$ (187)
Derivative Financial Instruments, Liabilities [Member] | Discounted Cash Flow [Member] | Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net derivative asset (liability)	$ (132)		$ (142)
Derivative Financial Instruments, Liabilities [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	2.15	2.10
Derivative Financial Instruments, Liabilities [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	3.65	2.88