EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 71.3	$ 75.0	$ 91.4
Interest Cost	188.0	185.5	227.0
Expected Return on Pension Plan Assets	(334.1)	(317.9)	(335.9)
Actuarial Loss	135.2	125.7	148.5
Prior Service Cost	4.5	3.6	3.7
Total Net Periodic Benefit Income	64.9	71.9	134.7
Capitalized Pension Expense	22.0	22.1	41.0
Amount included in other deferred debits			3.2
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	9.5	12.2	16.3
Interest Cost	27.1	32.9	47.2
Expected Return on Pension Plan Assets	(63.7)	(62.9)	(67.4)
Actuarial Loss	9.1	9.0	6.8
Prior Service Cost	(21.6)	(9.1)	(0.5)
Total Net Periodic Benefit Income	(39.6)	(17.9)	2.4
Capitalized Pension Expense	19.1	8.0	(0.1)
The Connecticut Light And Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	18.5	18.8	24.7
Interest Cost	41.6	41.6	51.1
Expected Return on Pension Plan Assets	(71.7)	(72.1)	(78.9)
Actuarial Loss	27.7	25.4	32.2
Prior Service Cost	1.5	1.5	1.5
Total Net Periodic Benefit Income	17.6	15.2	30.6
Intercompany Allocations	9.8	13.8	22.5
Capitalized Pension Expense	9.7	9.3	18.8
The Connecticut Light And Power Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.9	2.0	2.1
Interest Cost	5.3	5.3	7.2
Expected Return on Pension Plan Assets	(9.7)	(10.1)	(11.1)
Actuarial Loss	1.0	1.5	0.7
Prior Service Cost	1.1	0.5	0.0
Total Net Periodic Benefit Income	(0.4)	(0.8)	(1.1)
Intercompany Allocations	(0.7)	0.3	1.9
Capitalized Pension Expense	0.5	0.5	0.2
NSTAR Electric Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	15.5	16.3	19.2
Interest Cost	42.7	42.2	50.6
Expected Return on Pension Plan Assets	(87.6)	(85.1)	(88.9)
Actuarial Loss	41.1	39.9	42.2
Prior Service Cost	0.6	0.3	0.2
Total Net Periodic Benefit Income	12.3	13.6	23.3
Intercompany Allocations	9.1	11.4	18.0
Capitalized Pension Expense	7.6	8.0	13.3
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.7	3.4	5.8
Interest Cost	8.7	13.3	20.5
Expected Return on Pension Plan Assets	(28.6)	(28.1)	(29.8)
Actuarial Loss	3.4	3.3	2.3
Prior Service Cost	(17.0)	(7.1)	(0.2)
Total Net Periodic Benefit Income	(31.8)	(15.2)	(1.4)
Intercompany Allocations	(1.1)	(0.1)	1.1
Capitalized Pension Expense	16.2	6.7	0.4
Public Service Company Of New Hampshire | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	9.7	9.9	12.1
Interest Cost	21.2	20.7	24.3
Expected Return on Pension Plan Assets	(40.0)	(38.6)	(40.4)
Actuarial Loss	11.6	9.9	11.6
Prior Service Cost	0.5	0.5	0.5
Total Net Periodic Benefit Income	3.0	2.4	8.1
Intercompany Allocations	3.3	4.0	6.7
Capitalized Pension Expense	1.5	1.4	3.5
Public Service Company Of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.3	1.3	1.4
Interest Cost	3.0	2.9	3.9
Expected Return on Pension Plan Assets	(5.5)	(5.5)	(6.0)
Actuarial Loss	0.6	0.7	0.5
Prior Service Cost	0.6	0.2	0.0
Total Net Periodic Benefit Income	0.0	(0.4)	(0.2)
Intercompany Allocations	(0.5)	(0.1)	0.4
Capitalized Pension Expense	$ (0.2)	$ (0.1)	$ (0.2)