PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Components of Periodic Benefit Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts included in other deferred benefits		$ 0.8		$ 2.4
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 18.6	22.7	$ 56.6	68.7
Interest Cost	46.4	56.9	139.2	170.3
Expected Return on Plan Assets	(79.4)	(83.9)	(238.5)	(252.1)
Actuarial Loss	31.4	36.5	94.2	111.9
Prior Service Cost/(Credit)	0.9	0.9	2.6	2.7
Total Net Periodic Benefit Expense/(Income)	17.9	33.1	54.1	101.5
Capitalized PBOP Expense/(Income)	5.4	9.8	16.8	31.3
Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.0	4.1	9.2	10.3
Interest Cost	7.5	11.8	26.5	30.4
Expected Return on Plan Assets	(15.9)	(16.9)	(47.3)	(43.2)
Actuarial Loss	3.0	1.7	5.0	4.5
Prior Service Cost/(Credit)	(3.6)	(0.1)	(3.7)	(0.3)
Total Net Periodic Benefit Expense/(Income)	(6.0)	0.6	(10.3)	1.7
Capitalized PBOP Expense/(Income)	(2.6)	0.0	(4.6)	0.1
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	4.6	6.2	14.3	18.4
Interest Cost	10.2	12.9	31.2	38.4
Expected Return on Plan Assets	(18.0)	(19.8)	(54.2)	(59.1)
Actuarial Loss	6.3	8.0	19.2	24.2
Prior Service Cost/(Credit)	0.4	0.4	1.1	1.1
Total Net Periodic Benefit Expense/(Income)	3.5	7.7	11.6	23.0
Intercompany Allocations	3.5	5.8	10.3	18.0
Capitalized PBOP Expense/(Income)	2.2	4.7	7.1	14.1
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	0.6	0.5	1.4	1.6
Interest Cost	1.3	1.8	4.0	5.4
Expected Return on Plan Assets	(2.5)	(2.8)	(7.6)	(8.3)
Actuarial Loss	0.5	0.2	0.9	0.5
Prior Service Cost/(Credit)	0.2	0.0	0.2	0.0
Total Net Periodic Benefit Expense/(Income)	0.1	(0.3)	(1.1)	(0.8)
Intercompany Allocations	0.0	0.4	0.3	1.4
Capitalized PBOP Expense/(Income)	0.0	(0.1)	(0.5)	(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	3.7	9.9	11.2
Interest Cost	8.5	10.0	25.3	30.2
Expected Return on Plan Assets	(16.9)	(17.5)	(50.7)	(52.5)
Actuarial Loss	8.7	8.7	25.8	27.0
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.1
Total Net Periodic Benefit Expense/(Income)	3.6	4.9	10.3	16.0
Intercompany Allocations	2.2	3.4	6.7	10.3
Capitalized PBOP Expense/(Income)	2.0	2.7	5.7	8.6
NSTAR Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.6	1.3	2.5	4.0
Interest Cost	2.5	4.8	10.3	14.2
Expected Return on Plan Assets	(6.4)	(6.8)	(19.2)	(20.5)
Actuarial Loss	1.2	0.6	1.7	1.8
Prior Service Cost/(Credit)	(2.9)	(0.1)	(2.9)	(0.1)
Total Net Periodic Benefit Expense/(Income)	(5.0)	(0.2)	(7.6)	(0.6)
Intercompany Allocations	(0.1)	0.2	0.0	0.7
Capitalized PBOP Expense/(Income)	(2.2)	0.0	(3.3)	(0.1)
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	3.1	7.5	9.0
Interest Cost	5.1	6.1	15.4	18.1
Expected Return on Plan Assets	(9.6)	(10.1)	(28.9)	(30.3)
Actuarial Loss	2.5	2.9	7.5	8.8
Prior Service Cost/(Credit)	0.1	0.1	0.3	0.4
Total Net Periodic Benefit Expense/(Income)	0.6	2.1	1.8	6.0
Intercompany Allocations	1.0	1.6	3.0	5.0
Capitalized PBOP Expense/(Income)	0.4	0.8	1.0	2.6
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	0.4	0.3	0.9	1.0
Interest Cost	0.7	1.0	2.2	2.9
Expected Return on Plan Assets	(1.4)	(1.5)	(4.2)	(4.4)
Actuarial Loss	0.2	0.1	0.5	0.4
Prior Service Cost/(Credit)	0.1	0.0	0.1	0.0
Total Net Periodic Benefit Expense/(Income)	0.0	(0.1)	(0.5)	(0.1)
Intercompany Allocations	0.0	0.1	0.0	0.3
Capitalized PBOP Expense/(Income)	0.0	0.1	0.0	0.2
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	1.0	2.4	3.2
Interest Cost	2.1	2.6	6.3	7.8
Expected Return on Plan Assets	(4.4)	(4.7)	(13.1)	(14.2)
Actuarial Loss	1.3	1.6	4.1	4.8
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Total Net Periodic Benefit Expense/(Income)	(0.1)	0.6	(0.1)	1.8
Intercompany Allocations	0.6	1.1	1.9	3.3
Capitalized PBOP Expense/(Income)	0.1	0.5	0.3	1.4
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	0.3	0.3
Interest Cost	0.3	0.3	0.8	1.1
Expected Return on Plan Assets	(0.6)	(0.6)	(1.7)	(1.9)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.0	0.0	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(0.2)	(0.2)	(0.6)	(0.5)
Intercompany Allocations	0.0	0.1	0.0	0.3
Capitalized PBOP Expense/(Income)	$ (0.1)	$ 0.0	$ (0.3)	$ (0.1)