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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, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Related Party Transaction [Line Items] |  |  |  |  |
Related party transaction, expenses from transactions with related party | $ 6 | $ 7 | $ 12 | $ 13 |
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 | 1 | 2 | 3 | 4 |
Pine Needle LNG Company, LLC [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Related party transaction, expenses from transactions with related party | 2 | 2 | 4 | 4 |
Hardy Storage Company, LLC [Member] |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Related party transaction, expenses from transactions with related party | $ 3 | $ 3 | $ 5 | $ 5 |

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