EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (6,321.7)	$ (5,520.0)
Service Cost	(76.2)	(67.7)	$ (84.8)
Interest Cost	(177.8)	(219.0)	(196.4)
Actuarial Loss	(658.2)	(815.3)
Benefits Paid	279.3	273.0
Benefits Paid - Lump Sum	23.4	20.0
Benefits Paid - SERP	7.3	7.3
Employee Transfers	0.0	0.0
Increase due to acquisition of CMA	(121.4)	0.0
Benefit Obligation as of End of Year	(7,045.3)	(6,321.7)	(5,520.0)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	4,968.6	4,573.9
Employer Contributions	109.6	112.5
Actual Return on Plan Assets	512.3	575.2
Benefits Paid - Pension	(279.3)	(273.0)
Benefits Paid - Lump Sum	(23.4)	(20.0)
Employee Transfers	0.0	0.0
Increase due to acquisition of CMA	121.4	0.0
Fair Value of Plan Assets as of End of Year	5,409.2	4,968.6	4,573.9
Funded Status as of December 31st	(1,636.1)	(1,353.1)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(899.0)	(841.5)
Service Cost	(10.2)	(7.8)	(10.0)
Interest Cost	(24.6)	(32.7)	(30.7)
Actuarial Loss	(82.8)	(67.0)
Benefits Paid	50.2	50.0
Employee Transfers	0.0	0.0
Increase due to acquisition of CMA	(27.5)	0.0
Benefit Obligation as of End of Year	(993.9)	(899.0)	(841.5)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	935.9	849.6
Employer Contributions	1.9	9.3
Actual Return on Plan Assets	116.5	127.0
Benefits Paid - Pension	(50.2)	(50.0)
Employee Transfers	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,004.1	935.9	849.6
Funded Status as of December 31st	10.2	36.9
The Connecticut Light and Power Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,331.3)	(1,160.4)
Service Cost	(21.8)	(18.0)	(21.4)
Interest Cost	(37.3)	(45.7)	(41.8)
Actuarial Loss	(152.3)	(176.6)
Benefits Paid	63.6	60.2
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.3
Employee Transfers	1.5	8.9
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(1,477.3)	(1,331.3)	(1,160.4)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	986.2	918.4
Employer Contributions	23.2	24.0
Actual Return on Plan Assets	98.8	112.9
Benefits Paid - Pension	(63.6)	(60.2)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	(1.5)	(8.9)
Increase due to acquisition of CMA	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,043.1	986.2	918.4
Funded Status as of December 31st	(434.2)	(345.1)
The Connecticut Light and Power Company | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(172.7)	(161.7)
Service Cost	(1.7)	(1.4)	(1.9)
Interest Cost	(4.4)	(6.3)	(5.8)
Actuarial Loss	(8.6)	(13.4)
Benefits Paid	10.1	10.8
Employee Transfers	(1.3)	(0.7)
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(178.6)	(172.7)	(161.7)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	126.3	120.6
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	15.7	17.1
Benefits Paid - Pension	(10.1)	(10.8)
Employee Transfers	2.2	(0.6)
Fair Value of Plan Assets as of End of Year	134.1	126.3	120.6
Funded Status as of December 31st	(44.5)	(46.4)
NSTAR Electric Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,397.3)	(1,236.5)
Service Cost	(15.4)	(14.6)	(17.4)
Interest Cost	(38.6)	(49.0)	(43.5)
Actuarial Loss	(139.5)	(181.0)
Benefits Paid	59.4	67.1
Benefits Paid - Lump Sum	13.1	12.9
Benefits Paid - SERP	0.2	0.1
Employee Transfers	0.2	3.7
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(1,517.9)	(1,397.3)	(1,236.5)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	1,288.8	1,222.1
Employer Contributions	0.7	0.4
Actual Return on Plan Assets	128.3	150.0
Benefits Paid - Pension	(59.4)	(67.1)
Benefits Paid - Lump Sum	(13.1)	(12.9)
Employee Transfers	(0.2)	(3.7)
Increase due to acquisition of CMA	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,345.1	1,288.8	1,222.1
Funded Status as of December 31st	(172.8)	(108.5)
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(258.3)	(246.3)
Service Cost	(2.1)	(1.7)	(2.0)
Interest Cost	(6.6)	(9.5)	(8.7)
Actuarial Loss	(7.4)	(15.2)
Benefits Paid	14.9	15.4
Employee Transfers	(1.0)	(1.0)
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(260.5)	(258.3)	(246.3)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	424.4	379.1
Employer Contributions	0.0	6.0
Actual Return on Plan Assets	53.3	57.0
Benefits Paid - Pension	(14.9)	(15.4)
Employee Transfers	1.8	(2.3)
Fair Value of Plan Assets as of End of Year	464.6	424.4	379.1
Funded Status as of December 31st	204.1	166.1
Public Service Company of New Hampshire | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(692.6)	(610.7)
Service Cost	(8.2)	(7.1)	(11.2)
Interest Cost	(19.4)	(24.0)	(22.0)
Actuarial Loss	(62.1)	(84.5)
Benefits Paid	33.5	30.3
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.4	0.4
Employee Transfers	(0.3)	3.0
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(748.7)	(692.6)	(610.7)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	551.6	506.6
Employer Contributions	19.5	15.4
Actual Return on Plan Assets	55.8	62.9
Benefits Paid - Pension	(33.5)	(30.3)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	0.3	(3.0)
Increase due to acquisition of CMA	0.0	0.0
Fair Value of Plan Assets as of End of Year	593.7	551.6	506.6
Funded Status as of December 31st	(155.0)	(141.0)
Public Service Company of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(93.0)	(91.9)
Service Cost	(0.9)	(0.7)	(1.1)
Interest Cost	(2.8)	(3.4)	(3.4)
Actuarial Loss	(19.0)	(3.1)
Benefits Paid	6.1	5.6
Employee Transfers	0.1	0.5
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(109.5)	(93.0)	(91.9)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	76.0	71.2
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	9.3	10.0
Benefits Paid - Pension	(6.1)	(5.6)
Employee Transfers	0.2	0.4
Fair Value of Plan Assets as of End of Year	79.4	76.0	$ 71.2
Funded Status as of December 31st	$ (30.1)	$ (17.0)