PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 20.7	$ 17.4	$ 43.1	$ 36.4
Interest Cost	49.1	47.2	97.8	93.5
Expected Return on Plan Assets	(97.9)	(83.5)	(195.8)	(167.0)
Actuarial Loss	35.7	33.9	71.8	67.5
Prior Service Cost/(Credit)	2.1	1.1	4.1	2.0
Total Net Periodic Benefit Expense/(Income)	9.7	16.1	21.0	32.4
Capitalized Pension Expense/(Income)	5.9	5.5	13.4	10.9
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.5	2.4	5.0	4.7
Interest Cost	6.8	6.3	14.4	13.5
Expected Return on Plan Assets	(16.7)	(16.0)	(34.8)	(31.9)
Actuarial Loss	1.6	2.5	4.4	4.6
Prior Service Cost/(Credit)	(5.7)	(5.3)	(11.5)	(10.7)
Total Net Periodic Benefit Expense/(Income)	(11.5)	(10.1)	(22.5)	(19.8)
Capitalized Pension Expense/(Income)	0.8	(4.9)	1.6	(9.5)
The Connecticut Light and Power Company | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5.2	4.6	11.0	9.3
Interest Cost	10.5	10.5	20.9	20.9
Expected Return on Plan Assets	(19.4)	(17.8)	(40.2)	(36.0)
Actuarial Loss	7.1	6.8	14.8	13.9
Prior Service Cost/(Credit)	0.2	0.4	0.6	0.7
Total Net Periodic Benefit Expense/(Income)	3.6	4.5	7.1	8.8
Intercompany Allocations	1.5	2.4	3.0	5.0
Capitalized Pension Expense/(Income)	1.9	2.4	4.2	4.9
The Connecticut Light and Power Company | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.5	0.9	0.9
Interest Cost	1.5	1.2	2.9	2.7
Expected Return on Plan Assets	(2.6)	(2.4)	(5.2)	(4.8)
Actuarial Loss	0.4	0.3	0.8	0.5
Prior Service Cost/(Credit)	0.3	0.3	0.5	0.5
Total Net Periodic Benefit Expense/(Income)	0.0	(0.1)	(0.1)	(0.2)
Intercompany Allocations	(0.2)	(0.1)	(0.5)	(0.4)
Capitalized Pension Expense/(Income)	0.2	(0.1)	0.4	(0.2)
NSTAR Electric Company | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.3	3.8	9.0	7.8
Interest Cost	10.9	10.7	21.7	21.2
Expected Return on Plan Assets	(26.6)	(21.9)	(51.8)	(44.0)
Actuarial Loss	10.1	10.4	20.7	20.5
Prior Service Cost/(Credit)	0.1	0.2	0.1	0.4
Total Net Periodic Benefit Expense/(Income)	(1.2)	3.2	(0.3)	5.9
Intercompany Allocations	1.6	2.3	3.2	4.6
Capitalized Pension Expense/(Income)	1.9	2.0	4.0	3.7
NSTAR Electric Company | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.4	1.0	0.9
Interest Cost	2.2	2.0	4.4	4.3
Expected Return on Plan Assets	(8.1)	(7.2)	(16.2)	(14.3)
Actuarial Loss	0.4	0.8	1.1	1.7
Prior Service Cost/(Credit)	(4.3)	(4.3)	(8.5)	(8.5)
Total Net Periodic Benefit Expense/(Income)	(9.3)	(8.3)	(18.2)	(15.9)
Intercompany Allocations	(0.3)	(0.2)	(0.7)	(0.6)
Capitalized Pension Expense/(Income)	0.3	(4.2)	0.5	(8.1)
Public Service Company of New Hampshire | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.7	2.4	5.7	4.9
Interest Cost	5.5	5.3	10.9	10.5
Expected Return on Plan Assets	(10.8)	(10.0)	(21.8)	(19.9)
Actuarial Loss	3.3	3.0	6.5	5.8
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Total Net Periodic Benefit Expense/(Income)	0.8	0.8	1.5	1.5
Intercompany Allocations	0.5	0.8	1.0	1.7
Capitalized Pension Expense/(Income)	0.7	0.4	1.5	0.7
Public Service Company of New Hampshire | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3	0.6	0.6
Interest Cost	0.9	0.7	1.6	1.5
Expected Return on Plan Assets	(1.5)	(1.3)	(3.0)	(2.7)
Actuarial Loss	0.2	0.2	0.4	0.3
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.3
Total Net Periodic Benefit Expense/(Income)	0.0	0.0	(0.1)	0.0
Intercompany Allocations	(0.1)	(0.1)	(0.2)	(0.2)
Capitalized Pension Expense/(Income)	$ 0.1	$ 0.0	$ 0.2	$ 0.0