Investments in Unconsolidated Affiliates (Gas Utilities and Infrastructure Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments In Unconsolidated Affiliates [Line Items]
Investments in equity method unconsolidated affiliates	$ 1,175	$ 925
Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Investments in equity method unconsolidated affiliates	$ 763	566
Atlantic Coast Pipeline [Member] \| Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Equity Method Investment, Ownership Percentage	47.00%
Investments in equity method unconsolidated affiliates	$ 397	265
Sabal Trail Transmission Pipeline [Member] \| Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Equity Method Investment, Ownership Percentage	7.50%
Investments in equity method unconsolidated affiliates	$ 219	140
Constitution Pipeline [Member] \| Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Equity Method Investment, Ownership Percentage	24.00%
Investments in equity method unconsolidated affiliates	$ 81	82
Piedmont Natural Gas [Member]
Investments In Unconsolidated Affiliates [Line Items]
Investments in equity method unconsolidated affiliates	$ 61	212
Piedmont Natural Gas [Member] \| Cardinal Pipeline Company, LLC [Member] \| Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Equity Method Investment, Ownership Percentage	21.49%
Investments in equity method unconsolidated affiliates	$ 11	16
Piedmont Natural Gas [Member] \| Pine Needle LNG Company, LLC [Member] \| Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Equity Method Investment, Ownership Percentage	45.00%
Investments in equity method unconsolidated affiliates	$ 13	16
Piedmont Natural Gas [Member] \| Hardy Storage Company, LLC [Member] \| Gas Utilities and Infrastructure [Member]
Investments In Unconsolidated Affiliates [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Investments in equity method unconsolidated affiliates	$ 42	$ 47

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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+ References

No definition available.

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