PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 19.3	$ 22.7
Interest Cost	54.2	48.4
Expected Return on Pension Plan Assets	(92.3)	(98.0)
Actuarial Loss	35.8	36.0
Prior Service Cost/(Credit)	1.1	2.2
Total Net Periodic Benefit Expense	18.1	11.3
PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.1	2.7
Interest Cost	8.1	7.6
Expected Return on Pension Plan Assets	(16.6)	(18.1)
Actuarial Loss	2.5	2.6
Prior Service Cost/(Credit)	(5.8)	(5.9)
Total Net Periodic Benefit Expense	(9.7)	(11.1)
CL&P | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.8	5.7
Interest Cost	11.5	10.6
Expected Return on Pension Plan Assets	(18.8)	(20.8)
Actuarial Loss	8.1	7.4
Prior Service Cost/(Credit)	0.0	0.4
Total Net Periodic Benefit Expense	5.6	3.3
Intercompany Allocations	7.8	1.4
CL&P | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.5
Interest Cost	1.5	1.4
Expected Return on Pension Plan Assets	(2.3)	(2.6)
Actuarial Loss	0.4	0.3
Prior Service Cost/(Credit)	0.3	0.3
Total Net Periodic Benefit Expense	0.3	(0.1)
Intercompany Allocations	(0.1)	(0.3)
NSTAR Electric | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.9	4.7
Interest Cost	11.9	10.8
Expected Return on Pension Plan Assets	(24.4)	(25.2)
Actuarial Loss	9.4	10.5
Prior Service Cost/(Credit)	0.1	0.2
Total Net Periodic Benefit Expense	0.9	1.0
Intercompany Allocations	3.0	1.5
NSTAR Electric | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.5
Interest Cost	2.3	2.2
Expected Return on Pension Plan Assets	(7.5)	(8.1)
Actuarial Loss	0.9	0.7
Prior Service Cost/(Credit)	(4.2)	(4.3)
Total Net Periodic Benefit Expense	(8.1)	(9.0)
Intercompany Allocations	(0.2)	(0.3)
PSNH | Pension and SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.6	2.9
Interest Cost	6.2	5.3
Expected Return on Pension Plan Assets	(10.3)	(10.9)
Actuarial Loss	3.5	3.3
Prior Service Cost/(Credit)	0.0	0.1
Total Net Periodic Benefit Expense	2.0	0.7
Intercompany Allocations	2.4	0.5
PSNH | PBOP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.2	0.3
Interest Cost	0.9	0.8
Expected Return on Pension Plan Assets	(1.3)	(1.5)
Actuarial Loss	0.2	0.2
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Expense	0.1	(0.1)
Intercompany Allocations	$ (0.1)	$ (0.1)