Marketable Securities and Restricted Funds Schedule of Held to Maturity Securities - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Unrecognized Holding Gain	$ 0	$ 0
Held-to-maturity Securities, Unrecognized Holding Loss	$ 0	$ 0
M&N Canada six point nine zero percent senior secured notes due 2014 to 2019 \| Maritimes & Northeast Pipeline Limited Partnership
Schedule of Held-to-maturity Securities [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.90%
Held-to-maturity Securities [Member] \| Maximum
Schedule of Held-to-maturity Securities [Line Items]
Weighted Average Contractual Maturity Of Outstanding Securities	1 year

Weighted Average Contractual Maturity Of Outstanding Securities

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of unrecognized gain before loss on investments in held-to-maturity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

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- Definition

Amount of unrecognized loss before gain on investments in held-to-maturity securities. Excludes write-down for other than temporary impairment (OTTI).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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