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=6382943&loc=d3e33918-111571

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ 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=35755396&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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- 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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Equity Method Investments (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
CYAPC [Member] \| The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	34.50%
CYAPC [Member] \| NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	14.00%
CYAPC [Member] \| Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	5.00%
CYAPC [Member] \| Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	9.50%
YAEC [Member] \| The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	24.50%
YAEC [Member] \| NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	14.00%
YAEC [Member] \| Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.00%
YAEC [Member] \| Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.00%
MYAPC [Member] \| The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	12.00%
MYAPC [Member] \| NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	4.00%
MYAPC [Member] \| Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	5.00%
MYAPC [Member] \| Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	3.00%
Yankee Companies [Member] \| The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 1.2	$ 1.2
Yankee Companies [Member] \| NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	0.5	0.5
Yankee Companies [Member] \| Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	0.3	0.3
Yankee Companies [Member] \| Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	0.3	0.3
Energy Investment Fund [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	30.3	$ 17.8
Algonquin Gas Tansmsission LLC - Access Northeast [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 10.7