Related Party Transactions (Equity Method Investments) (Details) - Piedmont Natural Gas [Member] - Equity Method Investee [Member] - USD ($) $ in Millions	2 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Oct. 31, 2016	Oct. 31, 2015
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 6	$ 25	$ 29	$ 29
Accounts Payable, Related Parties, Current	2	2
Cardinal Pipeline Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	2	8	9	9
Pine Needle LNG Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	2	8	11	11
Hardy Storage Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 2	$ 9	$ 9	$ 9