Fair Value Measurements (Quantitative Level 3 Fair Value Disclosure - Liabilities) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) $ / MMBTU	Dec. 31, 2016 USD ($) $ / MMBTU	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net derivative asset (liability)	$ (135)	$ (166)	$ 2	$ 10
Piedmont Natural Gas [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Net derivative asset (liability)	(145)	(187)	$ (126)	$ (149)
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)	$ (145)	$ (187)
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	2.08	2.31
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	3.57	4.18