PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 17.5	$ 21.5	$ 52.7	$ 64.3
Interest Cost	38.6	32.5	115.8	97.5
Expected Return on Plan Assets	(130.7)	(109.5)	(393.1)	(328.0)
Actuarial Losses, net	28.4	60.7	87.5	183.1
Prior Service Cost/(Credit)	0.4	0.3	1.1	1.1
Total Net Periodic Benefit Plan Income	(45.8)	5.5	(136.0)	18.0
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.9	3.3	8.7	10.1
Interest Cost	5.0	4.3	15.1	12.9
Expected Return on Plan Assets	(22.5)	(19.8)	(67.5)	(59.3)
Actuarial Losses, net	0.0	2.0	0.0	5.9
Prior Service Cost/(Credit)	(5.4)	(5.3)	(16.2)	(15.9)
Total Net Periodic Benefit Plan Income	(20.0)	(15.5)	(59.9)	(46.3)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.7	5.6	13.9	17.4
Interest Cost	7.8	6.7	23.4	20.7
Expected Return on Plan Assets	(26.5)	(21.7)	(79.6)	(65.0)
Actuarial Losses, net	4.0	10.8	12.2	34.6
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(10.0)	1.4	(30.1)	7.7
Intercompany Income Allocations	(4.0)	2.2	(11.9)	5.8
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.5	1.5	1.7
Interest Cost	0.9	0.8	2.7	2.4
Expected Return on Plan Assets	(2.8)	(2.5)	(8.4)	(7.7)
Actuarial Losses, net	0.0	0.4	0.0	1.2
Prior Service Cost/(Credit)	0.3	0.3	0.8	0.8
Total Net Periodic Benefit Plan Income	(1.1)	(0.5)	(3.4)	(1.6)
Intercompany Income Allocations	(0.9)	(0.4)	(2.7)	(1.3)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.4	4.0	10.4	11.9
Interest Cost	8.2	6.7	24.5	20.1
Expected Return on Plan Assets	(32.1)	(27.1)	(96.2)	(81.1)
Actuarial Losses, net	8.1	15.3	24.7	46.1
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Total Net Periodic Benefit Plan Income	(12.3)	(1.0)	(36.3)	(2.7)
Intercompany Income Allocations	(3.2)	2.4	(9.3)	6.4
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.6	1.5	1.8
Interest Cost	1.3	1.1	3.9	3.3
Expected Return on Plan Assets	(10.6)	(9.2)	(31.8)	(27.7)
Actuarial Losses, net	0.0	0.5	0.0	1.6
Prior Service Cost/(Credit)	(4.2)	(4.3)	(12.7)	(12.7)
Total Net Periodic Benefit Plan Income	(13.0)	(11.3)	(39.1)	(33.7)
Intercompany Income Allocations	(0.9)	(0.5)	(2.7)	(1.5)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.7	2.2	5.2	6.6
Interest Cost	4.2	3.7	12.6	10.9
Expected Return on Plan Assets	(14.0)	(11.9)	(42.1)	(35.6)
Actuarial Losses, net	1.9	5.3	6.0	15.4
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Plan Income	(6.2)	(0.7)	(18.3)	(2.7)
Intercompany Income Allocations	(1.0)	0.7	(2.7)	1.9
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.8	0.9
Interest Cost	0.5	0.4	1.6	1.3
Expected Return on Plan Assets	(1.7)	(1.6)	(5.0)	(4.6)
Actuarial Losses, net	0.0	0.1	0.0	0.4
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Total Net Periodic Benefit Plan Income	(0.8)	(0.7)	(2.3)	(1.7)
Intercompany Income Allocations	$ (0.3)	$ (0.2)	$ (0.9)	$ (0.5)