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Common Stock (Equity Securities Offering) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Feb. 20, 2018
Dec. 31, 2018
Jun. 30, 2018
Oct. 31, 2016
Mar. 31, 2018
Dec. 31, 2018
Mar. 31, 2016
Forward Contract Indexed to Issuer's Equity [Line Items]              
Maximum value of shares to be issued under ATM program $ 1,000            
Stock Issued During Period, Shares, Dividend Reinvestment Plan           2.2  
Stock Issued During Period, Value, Dividend Reinvestment Plan           $ 174  
ATM [Member]              
Forward Contract Indexed to Issuer's Equity [Line Items]              
Settlement Alternatives, Shares   2.6 2.6     2.6  
Proceeds from Issuance or Sale of Equity   $ 195          
Equity Forwards [Member]              
Forward Contract Indexed to Issuer's Equity [Line Items]              
Shares offered         21.3    
Sales price         $ 75    
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share         $ 74.07    
Settlement Alternatives, Shares         21.3    
Shares, Issued   10.6 10.6     10.6  
Proceeds from Issuance or Sale of Equity   $ 766 $ 781        
Piedmont Natural Gas [Member]              
Forward Contract Indexed to Issuer's Equity [Line Items]              
Shares offered             10.6
Settlement Alternatives, Shares             10.6
Common stock issuances, including dividend reinvestment and employee benefits (in shares)       10.6      
Proceeds from Issuance or Sale of Equity       $ 723      
Wells Fargo Bank [Member] | ATM [Member]              
Forward Contract Indexed to Issuer's Equity [Line Items]              
Shares offered     1.3        
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share     $ 72.02        
Citibank [Member] | ATM [Member]              
Forward Contract Indexed to Issuer's Equity [Line Items]              
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share     $ 78.71