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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