Related Party Transactions (Equity Method Investments) (Details) - Piedmont Natural Gas [Member] - Equity Method Investee [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 7	$ 7	$ 13	$ 14
Accounts Payable, Related Parties, Current	2		2		$ 2
Cardinal Pipeline Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	2	2	4	4
Pine Needle LNG Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	2	2	4	5
Hardy Storage Company, LLC [Member]
Related Party Transaction [Line Items]
Related party transaction, expenses from transactions with related party	$ 3	$ 3	$ 5	$ 5