EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 70.1	$ 85.8	$ 76.2
Interest Cost	154.5	130.0	177.8
Expected Return on Plan Assets	(523.6)	(437.5)	(400.3)
Actuarial Loss	116.0	243.9	202.0
Prior Service Cost/(Credit)	1.4	1.4	1.2
Total Net Periodic Benefit Plan Expense/(Income)	$ (181.6)	$ 23.6	56.9
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income, Net	Other Income, Net
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 11.6	$ 13.5	10.2
Interest Cost	20.2	17.4	24.6
Expected Return on Plan Assets	(89.9)	(79.1)	(73.6)
Actuarial Loss	0.0	8.9	8.4
Prior Service Cost/(Credit)	(21.7)	(21.2)	(21.2)
Total Net Periodic Benefit Plan Expense/(Income)	$ (79.8)	$ (60.5)	(51.6)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income, Net	Other Income, Net
CL&P | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 18.7	$ 23.0	21.8
Interest Cost	31.3	27.3	37.3
Expected Return on Plan Assets	(106.3)	(86.8)	(79.2)
Actuarial Loss	16.2	45.5	39.2
Prior Service Cost/(Credit)	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(40.1)	9.0	19.1
Intercompany Income Allocations	(16.0)	8.0	9.1
CL&P | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.0	2.3	1.7
Interest Cost	3.7	3.2	4.4
Expected Return on Plan Assets	(11.4)	(10.3)	(9.9)
Actuarial Loss	0.0	1.8	1.1
Prior Service Cost/(Credit)	1.1	1.1	1.1
Total Net Periodic Benefit Plan Expense/(Income)	(4.6)	(1.9)	(1.6)
Intercompany Income Allocations	(3.7)	(1.6)	(1.1)
NSTAR Electric | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	13.8	15.8	15.4
Interest Cost	32.8	26.8	38.6
Expected Return on Plan Assets	(128.4)	(108.1)	(103.0)
Actuarial Loss	32.8	61.6	55.2
Prior Service Cost/(Credit)	0.3	0.3	0.3
Total Net Periodic Benefit Plan Expense/(Income)	(48.7)	(3.6)	6.5
Intercompany Income Allocations	(12.4)	8.8	8.9
NSTAR Electric | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.0	2.4	2.1
Interest Cost	5.3	4.4	6.6
Expected Return on Plan Assets	(42.4)	(36.9)	(34.0)
Actuarial Loss	0.0	2.4	2.5
Prior Service Cost/(Credit)	(17.0)	(17.0)	(17.0)
Total Net Periodic Benefit Plan Expense/(Income)	(52.1)	(44.7)	(39.8)
Intercompany Income Allocations	(3.6)	(1.9)	(1.4)
PSNH | Pension and SERP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	6.9	8.9	8.2
Interest Cost	16.9	14.5	19.4
Expected Return on Plan Assets	(56.1)	(47.5)	(44.7)
Actuarial Loss	7.9	20.7	15.6
Prior Service Cost/(Credit)	0.0	0.0	0.0
Total Net Periodic Benefit Plan Expense/(Income)	(24.4)	(3.4)	(1.5)
Intercompany Income Allocations	(3.6)	2.7	2.9
PSNH | PBOP
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.1	1.2	0.9
Interest Cost	2.2	1.8	2.8
Expected Return on Plan Assets	(6.7)	(6.1)	(5.7)
Actuarial Loss	0.0	0.7	0.8
Prior Service Cost/(Credit)	0.4	0.4	0.4
Total Net Periodic Benefit Plan Expense/(Income)	(3.0)	(2.0)	(0.8)
Intercompany Income Allocations	$ (1.2)	$ (0.6)	$ (0.5)