EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (5,080.1)	$ (5,486.2)
Plan Amendment	(9.0)	0.0
Service Cost	(75.0)	(91.4)	$ (79.9)
Interest Cost	(185.5)	(227.0)	(225.7)
Actuarial Gain/(Loss)	(151.8)	331.5
Benefits Paid - Pension	254.0	238.5
Benefits Paid - Lump Sum	0.0	149.5
Benefits Paid - SERP	5.1	5.0
Benefit Obligation as of End of Year	(5,242.3)	(5,080.1)	(5,486.2)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	3,905.4	4,126.5
Employer Contributions	146.2	154.6
Actual Return on Pension Plan Assets	278.4	12.3
Benefits Paid	(254.0)	(238.5)
Benefits Paid - Lump Sum	0.0	149.5
Fair Value of Pension Plan Assets as of End of Year	4,076.0	3,905.4	4,126.5
Funded Status as of December 31st	(1,166.3)	(1,174.7)
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,051.4)	(1,147.9)
Plan Amendment	244.0	0.0
Service Cost	(12.2)	(16.3)	(12.5)
Interest Cost	(32.9)	(47.2)	(49.5)
Actuarial Gain/(Loss)	(17.7)	106.0
Benefits Paid - Pension	60.2	54.0
Benefit Obligation as of End of Year	(810.0)	(1,051.4)	(1,147.9)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	812.2	862.6
Employer Contributions	12.5	7.9
Actual Return on Pension Plan Assets	51.3	(4.3)
Benefits Paid	(60.2)	(54.0)
Fair Value of Pension Plan Assets as of End of Year	815.8	812.2	862.6
Funded Status as of December 31st	5.8	(239.2)
The Connecticut Light And Power Company [Member] \| Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,157.6)	(1,230.1)
Plan Amendment	0.0	0.0
Change due to transfer of employees	8.8	(4.6)
Service Cost	(18.8)	(24.7)	(20.2)
Interest Cost	(41.6)	(51.1)	(50.5)
Actuarial Gain/(Loss)	(23.9)	77.8
Benefits Paid - Pension	62.6	60.2
Benefits Paid - Lump Sum	0.0	14.5
Benefits Paid - SERP	0.3	0.4
Benefit Obligation as of End of Year	(1,170.2)	(1,157.6)	(1,230.1)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	913.5	980.8
Change due to transfer of employees	(8.8)	4.6
Employer Contributions	0.4	0.0
Actual Return on Pension Plan Assets	63.0	2.8
Benefits Paid	(62.6)	(60.2)
Benefits Paid - Lump Sum	0.0	14.5
Fair Value of Pension Plan Assets as of End of Year	905.5	913.5	980.8
Funded Status as of December 31st	(264.7)	(244.1)
The Connecticut Light And Power Company [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(164.0)	(173.9)
Plan Amendment	(12.5)	0.0
Change due to transfer of employees	1.3	0.1
Service Cost	(2.0)	(2.1)	(2.2)
Interest Cost	(5.3)	(7.2)	(8.1)
Actuarial Gain/(Loss)	3.6	7.2
Benefits Paid - Pension	13.9	11.9
Benefit Obligation as of End of Year	(165.0)	(164.0)	(173.9)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	136.7	149.0
Change due to transfer of employees	(0.8)	0.0
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	7.2	(0.4)
Benefits Paid	(13.9)	(11.9)
Fair Value of Pension Plan Assets as of End of Year	129.2	136.7	149.0
Funded Status as of December 31st	(35.8)	(27.3)
NSTAR Electric Company [Member] \| Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(949.7)	(982.6)
Plan Amendment	(2.8)	0.0
Change due to transfer of employees	(0.6)	6.2
Service Cost	(13.2)	(14.9)	(13.6)
Interest Cost	(33.8)	(40.2)	(41.3)
Actuarial Gain/(Loss)	(33.3)	34.1
Benefits Paid - Pension	53.8	47.6
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.2	0.1
Benefit Obligation as of End of Year	(979.4)	(949.7)	(982.6)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	832.9	879.0
Change due to transfer of employees	0.6	(6.2)
Employer Contributions	28.4	5.0
Actual Return on Pension Plan Assets	59.2	2.7
Benefits Paid	(53.8)	(47.6)
Benefits Paid - Lump Sum	0.0	0.0
Fair Value of Pension Plan Assets as of End of Year	867.3	832.9	879.0
Funded Status as of December 31st	(112.1)	(116.8)
NSTAR Electric Company [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(412.8)	(468.7)
Plan Amendment	195.3	0.0
Change due to transfer of employees	0.3	2.3
Service Cost	(3.0)	(5.4)	(3.1)
Interest Cost	(12.2)	(19.0)	(19.4)
Actuarial Gain/(Loss)	(24.6)	59.1
Benefits Paid - Pension	20.3	18.9
Benefit Obligation as of End of Year	(236.7)	(412.8)	(468.7)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	320.3	336.5
Change due to transfer of employees	(0.3)	0.6
Employer Contributions	8.9	4.9
Actual Return on Pension Plan Assets	23.2	(2.8)
Benefits Paid	(20.3)	(18.9)
Fair Value of Pension Plan Assets as of End of Year	331.8	320.3	336.5
Funded Status as of December 31st	95.1	(92.5)
Public Service Company Of New Hampshire [Member] \| Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(547.6)	(580.7)
Plan Amendment	0.0	0.0
Change due to transfer of employees	2.4	(1.9)
Service Cost	(9.9)	(12.1)	(9.7)
Interest Cost	(20.7)	(24.3)	(23.8)
Actuarial Gain/(Loss)	(21.5)	38.9
Benefits Paid - Pension	24.9	23.2
Benefits Paid - Lump Sum	0.0	9.1
Benefits Paid - SERP	0.2	0.2
Benefit Obligation as of End of Year	(572.2)	(547.6)	(580.7)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	470.5	498.4
Change due to transfer of employees	(2.4)	1.9
Employer Contributions	17.1	1.0
Actual Return on Pension Plan Assets	33.7	1.5
Benefits Paid	(24.9)	(23.2)
Benefits Paid - Lump Sum	0.0	9.1
Fair Value of Pension Plan Assets as of End of Year	494.0	470.5	498.4
Funded Status as of December 31st	(78.2)	(77.1)
Public Service Company Of New Hampshire [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(88.5)	(91.8)
Plan Amendment	(6.7)	0.0
Change due to transfer of employees	0.3	(0.3)
Service Cost	(1.3)	(1.4)	(1.3)
Interest Cost	(2.9)	(3.9)	(4.3)
Actuarial Gain/(Loss)	3.6	3.6
Benefits Paid - Pension	5.8	5.3
Benefit Obligation as of End of Year	(89.7)	(88.5)	(91.8)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	75.8	80.9
Change due to transfer of employees	(0.2)	0.2
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	3.4	0.0
Benefits Paid	(5.8)	(5.3)
Fair Value of Pension Plan Assets as of End of Year	73.2	75.8	80.9
Funded Status as of December 31st	(16.5)	(12.7)
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefits Paid - Lump Sum	0.0
Change in Pension Plan Assets:
Benefits Paid - Lump Sum	0.0
Western Massachusetts Electric Company [Member] \| Pension Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(237.6)	(249.4)
Plan Amendment	0.0	0.0
Change due to transfer of employees	1.9	(1.3)
Service Cost	(3.1)	(4.3)	(3.5)
Interest Cost	(8.4)	(10.4)	(10.3)
Actuarial Gain/(Loss)	(3.9)	12.6
Benefits Paid - Pension	13.2	12.7
Benefits Paid - Lump Sum		2.5
Benefits Paid - SERP	0.0	0.0
Benefit Obligation as of End of Year	(237.9)	(237.6)	(249.4)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	220.8	234.0
Change due to transfer of employees	(1.9)	1.3
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	15.3	0.7
Benefits Paid	(13.2)	(12.7)
Benefits Paid - Lump Sum		2.5
Fair Value of Pension Plan Assets as of End of Year	221.0	220.8	234.0
Funded Status as of December 31st	(16.9)	(16.8)
Western Massachusetts Electric Company [Member] \| Other Postretirement Benefit Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(34.4)	(36.6)
Plan Amendment	(1.7)	0.0
Change due to transfer of employees	0.2	0.0
Service Cost	(0.4)	(0.4)	(0.4)
Interest Cost	(1.1)	(1.5)	(1.7)
Actuarial Gain/(Loss)	1.1	1.5
Benefits Paid - Pension	3.0	2.6
Benefit Obligation as of End of Year	(33.3)	(34.4)	(36.6)
Change in Pension Plan Assets:
Fair Value of Pension Plan Assets as of Beginning of Year	31.7	34.4
Change due to transfer of employees	(0.3)	0.0
Employer Contributions	0.0	0.0
Actual Return on Pension Plan Assets	1.4	(0.1)
Benefits Paid	(3.0)	(2.6)
Fair Value of Pension Plan Assets as of End of Year	29.8	31.7	$ 34.4
Funded Status as of December 31st	$ (3.5)	$ (2.7)

Change in assets due to the merger of the service companies or transfer to employees amoung operating companies

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Change in liability due to movement of emplyees due among operating companies or service companies

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The amount of benefit plan payments made in a lump sum to participants.

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Defined Benefit Plan payments related to the NU SERP plan

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The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

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Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

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1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

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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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The increase in the fair value of plan assets from contributions made by the employer.

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

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The amount of increase or decrease due to a change in the terms of an existing plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered.

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The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

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