EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 84.8	$ 71.3	$ 75.0
Interest Cost	196.4	188.0	185.5
Expected Return on Pension Plan Assets	(391.6)	(334.1)	(317.9)
Actuarial Loss	145.7	135.2	125.7
Prior Service Cost	4.3	4.5	3.6
Total Net Periodic Benefit Income	39.6	64.9	71.9
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	10.0	9.5	12.2
Interest Cost	30.7	27.1	32.9
Expected Return on Pension Plan Assets	(72.4)	(63.7)	(62.9)
Actuarial Loss	10.3	9.1	9.0
Prior Service Cost	(23.6)	(21.6)	(9.1)
Total Net Periodic Benefit Income	(45.0)	(39.6)	(17.9)
The Connecticut Light And Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	21.4	18.5	18.8
Interest Cost	41.8	41.6	41.6
Expected Return on Pension Plan Assets	(79.1)	(71.7)	(72.1)
Actuarial Loss	29.1	27.7	25.4
Prior Service Cost	1.1	1.5	1.5
Total Net Periodic Benefit Income	14.3	17.6	15.2
Intercompany Allocations	6.1	9.8	13.8
The Connecticut Light And Power Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.9	1.9	2.0
Interest Cost	5.8	5.3	5.3
Expected Return on Pension Plan Assets	(10.4)	(9.7)	(10.1)
Actuarial Loss	1.6	1.0	1.5
Prior Service Cost	1.1	1.1	0.5
Total Net Periodic Benefit Income	0.0	(0.4)	(0.8)
Intercompany Allocations	(1.0)	0.7	0.3
NSTAR Electric Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	17.4	15.5	16.3
Interest Cost	43.5	42.7	42.2
Expected Return on Pension Plan Assets	(104.9)	(87.6)	(85.1)
Actuarial Loss	41.1	41.1	39.9
Prior Service Cost	0.2	0.6	0.3
Total Net Periodic Benefit Income	(2.7)	12.3	13.6
Intercompany Allocations	6.5	9.1	11.4
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.0	1.7	3.4
Interest Cost	8.7	8.7	13.3
Expected Return on Pension Plan Assets	(32.5)	(28.6)	(28.1)
Actuarial Loss	2.3	3.4	3.3
Prior Service Cost	(16.9)	(17.0)	(7.1)
Total Net Periodic Benefit Income	(36.4)	(31.8)	(15.2)
Intercompany Allocations	(1.3)	1.1	(0.1)
Public Service Company Of New Hampshire | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	11.2	9.7	9.9
Interest Cost	22.0	21.2	20.7
Expected Return on Pension Plan Assets	(43.6)	(40.0)	(38.6)
Actuarial Loss	11.6	11.6	9.9
Prior Service Cost	0.4	0.5	0.5
Total Net Periodic Benefit Income	1.6	3.0	2.4
Intercompany Allocations	1.9	3.3	4.0
Public Service Company Of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.1	1.3	1.3
Interest Cost	3.4	3.0	2.9
Expected Return on Pension Plan Assets	(6.0)	(5.5)	(5.5)
Actuarial Loss	0.7	0.6	0.7
Prior Service Cost	0.5	0.6	0.2
Total Net Periodic Benefit Income	(0.3)	0.0	(0.4)
Intercompany Allocations	$ 0.4	$ 0.5	$ (0.1)