PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSION - Components of Periodic Benefit Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 22.7	$ 18.7
Interest Cost	48.4	46.3
Expected Return on Pension Plan Assets	(98.0)	(83.5)
Actuarial Loss	36.0	33.6
Prior Service Cost/(Credit)	2.2	1.1
Total Net Periodic Benefit Income	11.3	16.2
Capitalized PBOP Income/ Pension Expense	7.5	5.4
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.7	2.4
Interest Cost	7.6	7.2
Expected Return on Pension Plan Assets	(18.1)	(15.9)
Actuarial Loss	2.6	2.0
Prior Service Cost/(Credit)	(5.9)	(5.3)
Total Net Periodic Benefit Income	(11.1)	(9.6)
Capitalized PBOP Income/ Pension Expense	0.8	(4.6)
The Connecticut Light And Power Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	5.7	4.8
Interest Cost	10.6	10.5
Expected Return on Pension Plan Assets	(20.8)	(18.0)
Actuarial Loss	7.4	6.9
Prior Service Cost/(Credit)	0.4	0.4
Total Net Periodic Benefit Income	3.3	4.6
Intercompany Allocations	1.4	2.5
Capitalized PBOP Income/ Pension Expense	2.3	2.5
The Connecticut Light And Power Company | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.5
Interest Cost	1.4	1.4
Expected Return on Pension Plan Assets	(2.6)	(2.4)
Actuarial Loss	0.3	0.2
Prior Service Cost/(Credit)	0.3	0.3
Total Net Periodic Benefit Income	(0.1)	0.0
Intercompany Allocations	(0.3)	(0.3)
Capitalized PBOP Income/ Pension Expense	0.2	(0.1)
NSTAR Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.7	4.1
Interest Cost	10.8	10.4
Expected Return on Pension Plan Assets	(25.2)	(22.0)
Actuarial Loss	10.5	10.1
Prior Service Cost/(Credit)	0.2	0.2
Total Net Periodic Benefit Income	1.0	2.8
Intercompany Allocations	1.5	2.4
Capitalized PBOP Income/ Pension Expense	2.1	1.8
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.5	0.5
Interest Cost	2.2	2.4
Expected Return on Pension Plan Assets	(8.1)	(7.1)
Actuarial Loss	0.7	0.9
Prior Service Cost/(Credit)	(4.3)	(4.3)
Total Net Periodic Benefit Income	(9.0)	(7.6)
Intercompany Allocations	(0.3)	(0.3)
Capitalized PBOP Income/ Pension Expense	0.2	(3.8)
Public Service Company Of New Hampshire | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.9	2.5
Interest Cost	5.3	5.2
Expected Return on Pension Plan Assets	(10.9)	(9.9)
Actuarial Loss	3.3	2.8
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Income	0.7	0.7
Intercompany Allocations	0.5	0.8
Capitalized PBOP Income/ Pension Expense	0.8	0.3
Public Service Company Of New Hampshire | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.3
Interest Cost	0.8	0.8
Expected Return on Pension Plan Assets	(1.5)	(1.3)
Actuarial Loss	0.2	0.1
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Income	(0.1)	0.0
Intercompany Allocations	(0.1)	(0.1)
Capitalized PBOP Income/ Pension Expense	$ 0.1	$ 0.0