Guarantees (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($)
Variable Interest Entity, Not Primary Beneficiary \| MVP Joint Venture
Performance guarantee	$ 91
NORESCO Guarantees, Energy Savings
Undiscounted maximum aggregate payments related to guarantees	$ 115
Guarantee obligations term (in years)	12 years
Mountain Valley Pipeline \| Variable Interest Entity, Not Primary Beneficiary \| MVP Joint Venture
Percent of obligation to issue new guarantee	33.00%

Guarantor Obligations, Term of Extension

No definition available.

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- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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Balance Type:	credit
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- Definition

Indicates, if estimable, (as a percentage) the approximate extent to which the proceeds from liquidation of any assets held either as collateral or by third parties would be expected to cover the maximum potential amount of future payments under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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Period Type:	instant

- Details

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- Details

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- Details

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- Details

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- Details

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