PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Net Periodic Benefit Expense/(Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 19.6	$ 19.3
Interest Cost	44.3	54.2
Expected Return on Pension Plan Assets	(100.3)	(92.3)
Actuarial Loss	49.3	35.8
Prior Service Cost/(Credit)	0.3	1.1
Total Net Periodic Benefit Expense/(Income)	13.2	18.1
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.6	2.1
Interest Cost	6.1	8.1
Expected Return on Pension Plan Assets	(18.1)	(16.6)
Actuarial Loss	2.1	2.5
Prior Service Cost/(Credit)	(5.4)	(5.8)
Total Net Periodic Benefit Expense/(Income)	(12.7)	(9.7)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5.7	4.8
Interest Cost	9.5	11.5
Expected Return on Pension Plan Assets	(20.0)	(18.8)
Actuarial Loss	10.0	8.1
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	5.2	5.6
Intercompany Allocations	1.9	7.8
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.4
Interest Cost	1.2	1.5
Expected Return on Pension Plan Assets	(2.4)	(2.3)
Actuarial Loss	0.4	0.4
Prior Service Cost/(Credit)	0.2	0.3
Total Net Periodic Benefit Expense/(Income)	(0.1)	0.3
Intercompany Allocations	(0.4)	(0.1)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.8	3.9
Interest Cost	9.6	11.9
Expected Return on Pension Plan Assets	(25.8)	(24.4)
Actuarial Loss	13.3	9.4
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Expense/(Income)	1.0	0.9
Intercompany Allocations	1.9	3.0
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	0.4
Interest Cost	1.7	2.3
Expected Return on Pension Plan Assets	(8.5)	(7.5)
Actuarial Loss	0.7	0.9
Prior Service Cost/(Credit)	(4.2)	(4.2)
Total Net Periodic Benefit Expense/(Income)	(9.7)	(8.1)
Intercompany Allocations	(0.4)	(0.2)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.2	2.6
Interest Cost	4.9	6.2
Expected Return on Pension Plan Assets	(11.3)	(10.3)
Actuarial Loss	4.1	3.5
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(0.1)	2.0
Intercompany Allocations	0.6	2.4
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.2
Interest Cost	0.6	0.9
Expected Return on Pension Plan Assets	(1.4)	(1.3)
Actuarial Loss	0.1	0.2
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Expense/(Income)	(0.4)	0.1
Intercompany Allocations	$ (0.2)	$ (0.1)