Related Party Transactions (Equity Method Investments) (Details) - Piedmont Natural Gas [Member] - Equity Method Investee [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 6	$ 6	$ 18	$ 19
Cardinal Pipeline Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	2	2	5	6
Pine Needle LNG Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	2	2	6	6
Hardy Storage Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 2	$ 2	$ 7	$ 7