PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 20.9	$ 17.4	$ 64.2	$ 53.8
Interest Cost	49.2	47.2	147.1	140.7
Expected Return on Plan Assets	(97.8)	(83.5)	(293.8)	(250.5)
Actuarial Loss	35.7	33.9	107.6	101.3
Prior Service Cost/(Credit)	2.0	1.2	6.0	3.4
Total Net Periodic Benefit Expense/(Income)	10.0	16.2	31.1	48.7
Capitalized Pension Expense/(Income)	6.6	5.5	20.2	16.5
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.5	2.4	7.5	7.1
Interest Cost	7.7	6.8	23.0	20.3
Expected Return on Plan Assets	(18.2)	(16.0)	(54.2)	(47.8)
Actuarial Loss	2.6	2.2	7.7	6.9
Prior Service Cost/(Credit)	(6.0)	(5.3)	(17.6)	(16.1)
Total Net Periodic Benefit Expense/(Income)	(11.4)	(9.9)	(33.6)	(29.6)
Capitalized Pension Expense/(Income)	0.7	(4.8)	2.3	(14.3)
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	16.2	13.9
Interest Cost	10.5	10.5	31.4	31.3
Expected Return on Plan Assets	(19.4)	(17.8)	(59.7)	(53.9)
Actuarial Loss	7.1	6.8	21.9	20.7
Prior Service Cost/(Credit)	0.2	0.4	0.8	1.1
Total Net Periodic Benefit Expense/(Income)	3.6	4.5	10.6	13.1
Intercompany Allocations	1.5	2.4	4.5	7.4
Capitalized Pension Expense/(Income)	2.0	2.4	6.2	7.3
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	1.4	1.5
Interest Cost	1.5	1.3	4.4	4.0
Expected Return on Plan Assets	(2.6)	(2.4)	(7.8)	(7.3)
Actuarial Loss	0.4	0.2	1.2	0.7
Prior Service Cost/(Credit)	0.3	0.3	0.8	0.8
Total Net Periodic Benefit Expense/(Income)	0.0	(0.1)	0.0	(0.3)
Intercompany Allocations	(0.3)	(0.2)	(0.8)	(0.5)
Capitalized Pension Expense/(Income)	0.1	(0.1)	0.5	(0.4)
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	13.1	11.7
Interest Cost	10.9	10.7	32.6	31.9
Expected Return on Plan Assets	(26.6)	(21.9)	(78.3)	(65.8)
Actuarial Loss	10.1	10.4	31.0	30.9
Prior Service Cost/(Credit)	0.1	0.2	0.2	0.4
Total Net Periodic Benefit Expense/(Income)	(1.2)	3.2	(1.4)	9.1
Intercompany Allocations	1.6	2.3	4.8	6.9
Capitalized Pension Expense/(Income)	1.8	2.0	5.8	5.7
NSTAR Electric Company | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.4	1.6	1.4
Interest Cost	2.2	2.2	6.5	6.5
Expected Return on Plan Assets	(8.1)	(7.2)	(24.4)	(21.6)
Actuarial Loss	0.5	0.9	1.7	2.6
Prior Service Cost/(Credit)	(4.3)	(4.3)	(12.7)	(12.8)
Total Net Periodic Benefit Expense/(Income)	(9.2)	(8.0)	(27.3)	(23.9)
Intercompany Allocations	(0.3)	(0.2)	(1.0)	(0.8)
Capitalized Pension Expense/(Income)	0.2	(4.1)	0.7	(12.1)
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	8.4	7.3
Interest Cost	5.5	5.3	16.4	15.9
Expected Return on Plan Assets	(10.8)	(10.0)	(32.6)	(29.9)
Actuarial Loss	3.3	3.0	9.8	8.7
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.4
Total Net Periodic Benefit Expense/(Income)	0.8	0.8	2.2	2.4
Intercompany Allocations	0.5	0.8	1.4	2.5
Capitalized Pension Expense/(Income)	0.8	0.4	2.3	1.1
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	0.9	1.0
Interest Cost	0.9	0.8	2.5	2.3
Expected Return on Plan Assets	(1.5)	(1.4)	(4.5)	(4.1)
Actuarial Loss	0.2	0.1	0.6	0.4
Prior Service Cost/(Credit)	0.1	0.2	0.4	0.4
Total Net Periodic Benefit Expense/(Income)	0.0	0.0	(0.1)	0.0
Intercompany Allocations	(0.1)	(0.1)	(0.3)	(0.3)
Capitalized Pension Expense/(Income)	$ 0.1	$ 0.0	$ 0.3	$ 0.0