Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Piedmont Natural Gas [Member] \| Equity Method Investee [Member]
Related Party Transaction [Line Items]
Accounts payable, related parties	$ 2	$ 2

Amount for accounts payable to related parties. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Name:	us-gaap_AccountsPayableRelatedPartiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant