ASSET RETIREMENT OBLIGATIONS (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$ 426.3	$ 424.9
Asset Retirement Obligation, Liabilities Incurred	6.6	1.3
Asset Retirement Obligation, Liabilities Settled	(18.2)	(19.5)
Asset Retirement Obligation, Accretion Expense	26.5	25.1
Asset Retirement Obligation, Revision of Estimate	(11.1)	(5.5)
Asset Retirement Obligation, Ending Balance	430.1	426.3
The Connecticut Light And Power Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	35.3	35.0
Asset Retirement Obligation, Liabilities Settled	0.0	(1.1)
Asset Retirement Obligation, Accretion Expense	2.2	1.9
Asset Retirement Obligation, Revision of Estimate	(3.7)	(0.5)
Asset Retirement Obligation, Ending Balance	33.8	35.3
NSTAR Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	34.3	32.8
Asset Retirement Obligation, Liabilities Incurred	6.2
Asset Retirement Obligation, Liabilities Settled	(1.5)	0.0
Asset Retirement Obligation, Accretion Expense	1.8	1.5
Asset Retirement Obligation, Revision of Estimate	(5.5)	0.0
Asset Retirement Obligation, Ending Balance	35.3	34.3
Public Service Company Of New Hampshire [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	20.6	19.5
Asset Retirement Obligation, Liabilities Incurred	0.4	0.0
Asset Retirement Obligation, Accretion Expense	1.3	1.1
Asset Retirement Obligation, Revision of Estimate	(0.7)
Asset Retirement Obligation, Ending Balance	21.6	20.6
Western Massachusetts Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	5.9	4.5
Asset Retirement Obligation, Liabilities Incurred	0.0	1.1
Asset Retirement Obligation, Liabilities Settled	(0.1)
Asset Retirement Obligation, Accretion Expense	0.4	0.3
Asset Retirement Obligation, Revision of Estimate	(0.5)
Asset Retirement Obligation, Ending Balance	5.7	5.9
CYAPC and YAEC [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	317.3
Asset Retirement Obligation, Ending Balance	$ 319.1	$ 317.3

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Asset Retirement Obligation
-URI http://asc.fasb.org/extlink&oid=6505190

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Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

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Amount of asset retirement obligations incurred during the period.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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