PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 16.2	$ 20.9	$ 51.6	$ 64.2
Interest Cost	54.8	49.2	163.8	147.1
Expected Return on Pension Plan Assets	(91.6)	(97.8)	(275.4)	(293.8)
Actuarial Loss	35.6	35.7	107.9	107.6
Prior Service Cost/(Credit)	0.3	2.0	0.9	6.0
Total Net Periodic Benefit Expense/(Income)	15.3	10.0	48.8	31.1
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.0	2.5	5.9	7.5
Interest Cost	8.1	7.7	24.5	23.0
Expected Return on Pension Plan Assets	(16.7)	(18.2)	(50.1)	(54.2)
Actuarial Loss	2.1	2.6	6.2	7.7
Prior Service Cost/(Credit)	(5.8)	(6.0)	(17.6)	(17.6)
Total Net Periodic Benefit Expense/(Income)	(10.3)	(11.4)	(31.1)	(33.6)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.4	5.2	13.6	16.2
Interest Cost	11.4	10.5	34.4	31.4
Expected Return on Pension Plan Assets	(18.1)	(19.4)	(55.1)	(59.7)
Actuarial Loss	6.3	7.1	20.6	21.9
Prior Service Cost/(Credit)	0.0	0.2	0.0	0.8
Total Net Periodic Benefit Expense/(Income)	4.0	3.6	13.5	10.6
Intercompany Allocations	2.0	1.5	6.5	4.5
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.4	1.1	1.4
Interest Cost	1.6	1.5	4.7	4.4
Expected Return on Pension Plan Assets	(2.2)	(2.6)	(6.9)	(7.8)
Actuarial Loss	0.3	0.4	1.0	1.2
Prior Service Cost/(Credit)	0.2	0.3	0.8	0.8
Total Net Periodic Benefit Expense/(Income)	0.2	0.0	0.7	0.0
Intercompany Allocations	(0.2)	(0.3)	(0.6)	(0.8)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.6	4.3	11.0	13.1
Interest Cost	12.3	10.9	36.6	32.6
Expected Return on Pension Plan Assets	(24.2)	(26.6)	(72.9)	(78.3)
Actuarial Loss	11.8	10.1	33.1	31.0
Prior Service Cost/(Credit)	0.0	0.1	0.2	0.2
Total Net Periodic Benefit Expense/(Income)	3.5	(1.2)	8.0	(1.4)
Intercompany Allocations	1.9	1.6	6.1	4.8
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.5	1.3	1.6
Interest Cost	2.4	2.2	7.1	6.5
Expected Return on Pension Plan Assets	(7.5)	(8.1)	(22.6)	(24.4)
Actuarial Loss	0.8	0.5	2.5	1.7
Prior Service Cost/(Credit)	(4.2)	(4.3)	(12.7)	(12.7)
Total Net Periodic Benefit Expense/(Income)	(8.1)	(9.2)	(24.4)	(27.3)
Intercompany Allocations	(0.3)	(0.3)	(0.9)	(1.0)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	1.5	2.7	5.6	8.4
Interest Cost	6.0	5.5	18.2	16.4
Expected Return on Pension Plan Assets	(10.1)	(10.8)	(30.6)	(32.6)
Actuarial Loss	2.3	3.3	8.2	9.8
Prior Service Cost/(Credit)	0.0	0.1	0.0	0.2
Total Net Periodic Benefit Expense/(Income)	(0.3)	0.8	1.4	2.2
Intercompany Allocations	0.5	0.5	1.8	1.4
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.3	0.5	0.9
Interest Cost	0.8	0.9	2.5	2.5
Expected Return on Pension Plan Assets	(1.3)	(1.5)	(4.0)	(4.5)
Actuarial Loss	0.1	0.2	0.3	0.6
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.4
Total Net Periodic Benefit Expense/(Income)	(0.1)	0.0	(0.4)	(0.1)
Intercompany Allocations	$ (0.1)	$ (0.1)	$ (0.3)	$ (0.3)