PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Components of Periodic Benefit Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 18.7	$ 19.4
Interest Cost	46.3	46.5
Expected Return on Pension Plan Assets	(83.5)	(79.6)
Actuarial Loss	33.6	31.5
Prior Service Cost/(Credit)	1.1	0.9
Total Net Periodic Benefit Income	16.2	18.7
Capitalized PBOP Income	5.4	6.1
Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.4	3.1
Interest Cost	7.2	9.7
Expected Return on Pension Plan Assets	(15.9)	(15.7)
Actuarial Loss	2.0	1.1
Prior Service Cost/(Credit)	(5.3)	(0.1)
Total Net Periodic Benefit Income	(9.6)	(1.9)
Capitalized PBOP Income	(4.6)	(0.9)
The Connecticut Light And Power Company [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	4.8	5.0
Interest Cost	10.5	10.6
Expected Return on Pension Plan Assets	(18.0)	(18.2)
Actuarial Loss	6.9	6.7
Prior Service Cost/(Credit)	0.4	0.4
Total Net Periodic Benefit Income	4.6	4.5
Intercompany Allocations	2.5	3.3
Capitalized PBOP Income	2.5	2.7
The Connecticut Light And Power Company [Member] | Other Postretirement Benefit Plan [Member]
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	(2.4)	(2.6)
Actuarial Loss	0.2	0.2
Prior Service Cost/(Credit)	0.3	0.0
Total Net Periodic Benefit Income	0.0	(0.5)
Intercompany Allocations	(0.3)	0.2
Capitalized PBOP Income	(0.1)	(0.2)
NSTAR Electric Company [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.3	3.4
Interest Cost	8.3	8.3
Expected Return on Pension Plan Assets	(17.6)	(16.9)
Actuarial Loss	8.6	8.4
Prior Service Cost/(Credit)	0.1	0.0
Total Net Periodic Benefit Income	2.7	3.2
Intercompany Allocations	1.9	2.2
Capitalized PBOP Income	1.7	1.8
NSTAR Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.4	0.9
Interest Cost	2.1	4.0
Expected Return on Pension Plan Assets	(6.6)	(6.4)
Actuarial Loss	0.9	0.2
Prior Service Cost/(Credit)	(4.3)	0.0
Total Net Periodic Benefit Income	(7.5)	(1.3)
Intercompany Allocations	(0.3)	0.1
Capitalized PBOP Income	(3.8)	(0.6)
Public Service Company Of New Hampshire [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.5	2.5
Interest Cost	5.2	5.1
Expected Return on Pension Plan Assets	(9.9)	(9.7)
Actuarial Loss	2.8	2.5
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Income	0.7	0.5
Intercompany Allocations	0.8	1.0
Capitalized PBOP Income	0.3	0.3
Public Service Company Of New Hampshire [Member] | Other Postretirement Benefit Plan [Member]
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	(1.3)	(1.4)
Actuarial Loss	0.1	0.1
Prior Service Cost/(Credit)	0.1	0.0
Total Net Periodic Benefit Income	0.0	(0.2)
Intercompany Allocations	(0.1)	0.0
Capitalized PBOP Income	0.0	0.0
Western Massachusetts Electric Company [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.8	0.9
Interest Cost	2.1	2.1
Expected Return on Pension Plan Assets	(4.4)	(4.4)
Actuarial Loss	1.5	1.4
Prior Service Cost/(Credit)	0.1	0.1
Total Net Periodic Benefit Income	0.1	0.1
Intercompany Allocations	0.5	0.6
Capitalized PBOP Income	0.1	0.2
Western Massachusetts Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.1	0.1
Interest Cost	0.3	0.3
Expected Return on Pension Plan Assets	(0.5)	(0.6)
Actuarial Loss	0.0	0.0
Prior Service Cost/(Credit)	0.0	0.0
Total Net Periodic Benefit Income	(0.1)	(0.2)
Intercompany Allocations	0.0	0.0
Capitalized PBOP Income	$ 0.0	$ (0.1)