EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 67.7	$ 84.8	$ 71.3
Interest Cost	219.0	196.4	188.0
Expected Return on Plan Assets	(367.1)	(391.6)	(334.1)
Actuarial Loss	143.2	145.7	135.2
Prior Service Cost/(Credit)	0.9	4.3	4.5
Total Net Periodic Benefit Income	63.7	39.6	64.9
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	7.8	10.0	9.5
Interest Cost	32.7	30.7	27.1
Expected Return on Plan Assets	(66.8)	(72.4)	(63.7)
Actuarial Loss	8.3	10.3	9.1
Prior Service Cost/(Credit)	(23.5)	(23.6)	(21.6)
Total Net Periodic Benefit Income	(41.5)	(45.0)	(39.6)
CL&P | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	18.0	21.4	18.5
Interest Cost	45.7	41.8	41.6
Expected Return on Plan Assets	(73.2)	(79.1)	(71.7)
Actuarial Loss	26.9	29.1	27.7
Prior Service Cost/(Credit)	0.0	1.1	1.5
Total Net Periodic Benefit Income	17.4	14.3	17.6
Intercompany Allocations	8.5	6.1	9.8
CL&P | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.4	1.9	1.9
Interest Cost	6.3	5.8	5.3
Expected Return on Plan Assets	(9.2)	(10.4)	(9.7)
Actuarial Loss	1.3	1.6	1.0
Prior Service Cost/(Credit)	1.1	1.1	1.1
Total Net Periodic Benefit Income	0.9	0.0	(0.4)
Intercompany Allocations	(0.9)	(1.0)	(0.7)
NSTAR Electric | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	14.6	17.4	15.5
Interest Cost	49.0	43.5	42.7
Expected Return on Plan Assets	(97.1)	(104.9)	(87.6)
Actuarial Loss	44.7	41.1	41.1
Prior Service Cost/(Credit)	0.3	0.2	0.6
Total Net Periodic Benefit Income	11.5	(2.7)	12.3
Intercompany Allocations	8.0	6.5	9.1
NSTAR Electric | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.7	2.0	1.7
Interest Cost	9.5	8.7	8.7
Expected Return on Plan Assets	(30.2)	(32.5)	(28.6)
Actuarial Loss	3.3	2.3	3.4
Prior Service Cost/(Credit)	(16.9)	(16.9)	(17.0)
Total Net Periodic Benefit Income	(32.6)	(36.4)	(31.8)
Intercompany Allocations	(1.2)	(1.3)	(1.1)
PSNH | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	7.1	11.2	9.7
Interest Cost	24.0	22.0	21.2
Expected Return on Plan Assets	(40.7)	(43.6)	(40.0)
Actuarial Loss	10.6	11.6	11.6
Prior Service Cost/(Credit)	0.0	0.4	0.5
Total Net Periodic Benefit Income	1.0	1.6	3.0
Intercompany Allocations	2.3	1.9	3.3
PSNH | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	0.7	1.1	1.3
Interest Cost	3.4	3.4	3.0
Expected Return on Plan Assets	(5.4)	(6.0)	(5.5)
Actuarial Loss	0.3	0.7	0.6
Prior Service Cost/(Credit)	0.4	0.5	0.6
Total Net Periodic Benefit Income	(0.6)	(0.3)	0.0
Intercompany Allocations	$ (0.4)	$ (0.4)	$ (0.5)