EMPLOYEE BENEFITS - Schedule of Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	$ (7,045.3)	$ (6,321.7)
Service Cost	(85.8)	(76.2)	$ (67.7)
Interest Cost	(130.0)	(177.8)	(219.0)
Actuarial Gain/(Loss)	177.1	(658.2)
Benefits Paid	309.5	279.3
Benefits Paid - Lump Sum	34.7	23.4
Benefits Paid - SERP	10.1	7.3
Employee Transfers	0.0	0.0
Increase due to acquisition of CMA	0.0	(121.4)
Benefit Obligation as of End of Year	(6,729.7)	(7,045.3)	(6,321.7)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	5,409.2	4,968.6
Employer Contributions	180.0	109.6
Actual Return on Plan Assets	1,250.5	512.3
Benefits Paid - Pension	(309.5)	(279.3)
Benefits Paid - Lump Sum	(34.7)	(23.4)
Employee Transfers	0.0	0.0
Increase due to acquisition of CMA	0.0	121.4
Fair Value of Plan Assets as of End of Year	6,495.5	5,409.2	4,968.6
Funded Status as of December 31st	(234.2)	(1,636.1)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(993.9)	(899.0)
Service Cost	(13.5)	(10.2)	(7.8)
Interest Cost	(17.4)	(24.6)	(32.7)
Actuarial Gain/(Loss)	81.4	(82.8)
Benefits Paid	51.7	50.2
Employee Transfers	0.0	0.0
Increase due to acquisition of CMA	7.4	(27.5)
Benefit Obligation as of End of Year	(884.3)	(993.9)	(899.0)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	1,004.1	935.9
Employer Contributions	2.3	1.9
Actual Return on Plan Assets	183.2	116.5
Benefits Paid - Pension	(51.3)	(50.2)
Employee Transfers	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,138.3	1,004.1	935.9
Funded Status as of December 31st	254.0	10.2
The Connecticut Light and Power Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,477.3)	(1,331.3)
Service Cost	(23.0)	(21.8)	(18.0)
Interest Cost	(27.3)	(37.3)	(45.7)
Actuarial Gain/(Loss)	127.8	(152.3)
Benefits Paid	64.6	63.6
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.3	0.3
Employee Transfers	4.0	1.5
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(1,330.9)	(1,477.3)	(1,331.3)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	1,043.1	986.2
Employer Contributions	98.9	23.2
Actual Return on Plan Assets	250.4	98.8
Benefits Paid - Pension	(64.6)	(63.6)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	(4.0)	(1.5)
Increase due to acquisition of CMA	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,323.8	1,043.1	986.2
Funded Status as of December 31st	(7.1)	(434.2)
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	(178.6)	(172.7)
Service Cost	(2.3)	(1.7)	(1.4)
Interest Cost	(3.2)	(4.4)	(6.3)
Actuarial Gain/(Loss)	5.8	(8.6)
Benefits Paid	10.9	10.1
Employee Transfers	1.9	(1.3)
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(165.5)	(178.6)	(172.7)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	134.1	126.3
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	24.1	15.7
Benefits Paid - Pension	(10.9)	(10.1)
Employee Transfers	(1.6)	2.2
Fair Value of Plan Assets as of End of Year	145.7	134.1	126.3
Funded Status as of December 31st	(19.8)	(44.5)
NSTAR Electric Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(1,517.9)	(1,397.3)
Service Cost	(15.8)	(15.4)	(14.6)
Interest Cost	(26.8)	(38.6)	(49.0)
Actuarial Gain/(Loss)	20.8	(139.5)
Benefits Paid	68.7	59.4
Benefits Paid - Lump Sum	15.6	13.1
Benefits Paid - SERP	0.2	0.2
Employee Transfers	6.8	0.2
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(1,448.4)	(1,517.9)	(1,397.3)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	1,345.1	1,288.8
Employer Contributions	30.0	0.7
Actual Return on Plan Assets	312.0	128.3
Benefits Paid - Pension	(68.7)	(59.4)
Benefits Paid - Lump Sum	(15.6)	(13.1)
Employee Transfers	(6.8)	(0.2)
Increase due to acquisition of CMA	0.0	0.0
Fair Value of Plan Assets as of End of Year	1,596.0	1,345.1	1,288.8
Funded Status as of December 31st	147.6	(172.8)
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(260.5)	(258.3)
Service Cost	(2.4)	(2.1)	(1.7)
Interest Cost	(4.4)	(6.6)	(9.5)
Actuarial Gain/(Loss)	11.5	(7.4)
Benefits Paid	16.3	14.9
Employee Transfers	1.1	(1.0)
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(238.4)	(260.5)	(258.3)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	464.6	424.4
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	84.2	53.3
Benefits Paid - Pension	(16.3)	(14.9)
Employee Transfers	(2.5)	1.8
Fair Value of Plan Assets as of End of Year	530.0	464.6	424.4
Funded Status as of December 31st	291.6	204.1
Public Service Company of New Hampshire | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation as of Beginning of Year	(748.7)	(692.6)
Service Cost	(8.9)	(8.2)	(7.1)
Interest Cost	(14.5)	(19.4)	(24.0)
Actuarial Gain/(Loss)	14.7	(62.1)
Benefits Paid	34.7	33.5
Benefits Paid - Lump Sum	0.0	0.0
Benefits Paid - SERP	0.4	0.4
Employee Transfers	1.3	(0.3)
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(721.0)	(748.7)	(692.6)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	593.7	551.6
Employer Contributions	0.0	19.5
Actual Return on Plan Assets	136.9	55.8
Benefits Paid - Pension	(34.7)	(33.5)
Benefits Paid - Lump Sum	0.0	0.0
Employee Transfers	(1.3)	0.3
Increase due to acquisition of CMA	0.0	0.0
Fair Value of Plan Assets as of End of Year	694.6	593.7	551.6
Funded Status as of December 31st	(26.4)	(155.0)
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	(109.5)	(93.0)
Service Cost	(1.2)	(0.9)	(0.7)
Interest Cost	(1.8)	(2.8)	(3.4)
Actuarial Gain/(Loss)	14.6	(19.0)
Benefits Paid	5.6	6.1
Employee Transfers	0.0	0.1
Increase due to acquisition of CMA	0.0	0.0
Benefit Obligation as of End of Year	(92.3)	(109.5)	(93.0)
Change in Pension Plan Assets:
Fair Value of Plan Assets as of Beginning of Year	79.4	76.0
Employer Contributions	0.0	0.0
Actual Return on Plan Assets	14.2	9.3
Benefits Paid - Pension	(5.6)	(6.1)
Employee Transfers	0.0	0.2
Fair Value of Plan Assets as of End of Year	88.0	79.4	$ 76.0
Funded Status as of December 31st	$ (4.3)	$ (30.1)