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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 17.4	$ 18.6	$ 53.8	$ 56.6
Interest Cost	47.2	46.4	140.7	139.2
Expected Return on Pension Plan Assets	(83.5)	(79.4)	(250.5)	(238.5)
Actuarial Loss	33.9	31.4	101.3	94.2
Prior Service Cost/(Credit)	1.2	0.9	3.4	2.6
Total Net Periodic Benefit Income	16.2	17.9	48.7	54.1
Capitalized PBOP Income/ Pension Expense	5.5	5.4	16.5	16.8
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.4	3.0	7.1	9.2
Interest Cost	6.8	7.5	20.3	26.5
Expected Return on Pension Plan Assets	(16.0)	(15.9)	(47.8)	(47.3)
Actuarial Loss	2.2	3.0	6.9	5.0
Prior Service Cost/(Credit)	(5.3)	(3.6)	(16.1)	(3.7)
Total Net Periodic Benefit Income	(9.9)	(6.0)	(29.6)	(10.3)
Capitalized PBOP Income/ Pension Expense	(4.8)	(2.6)	(14.3)	(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	4.6	4.6	13.9	14.3
Interest Cost	10.5	10.2	31.3	31.2
Expected Return on Pension Plan Assets	(17.8)	(18.0)	(53.9)	(54.2)
Actuarial Loss	6.8	6.3	20.7	19.2
Prior Service Cost/(Credit)	0.4	0.4	1.1	1.1
Total Net Periodic Benefit Income	4.5	3.5	13.1	11.6
Intercompany Allocations	2.4	3.5	7.4	10.3
Capitalized PBOP Income/ Pension Expense	2.4	2.2	7.3	7.1
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	0.6	1.5	1.4
Interest Cost	1.3	1.3	4.0	4.0
Expected Return on Pension Plan Assets	(2.4)	(2.5)	(7.3)	(7.6)
Actuarial Loss	0.2	0.5	0.7	0.9
Prior Service Cost/(Credit)	0.3	0.2	0.8	0.2
Total Net Periodic Benefit Income	(0.1)	0.1	(0.3)	(1.1)
Intercompany Allocations	(0.2)	0.0	(0.5)	0.3
Capitalized PBOP Income/ Pension Expense	(0.1)	0.0	(0.4)	(0.5)
NSTAR Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	3.1	3.3	9.4	9.9
Interest Cost	8.6	8.5	25.6	25.3
Expected Return on Pension Plan Assets	(17.5)	(16.9)	(52.5)	(50.7)
Actuarial Loss	8.9	8.7	26.4	25.8
Prior Service Cost/(Credit)	0.1	0.0	0.2	0.0
Total Net Periodic Benefit Income	3.2	3.6	9.1	10.3
Intercompany Allocations	1.8	2.2	5.5	6.7
Capitalized PBOP Income/ Pension Expense	1.9	2.0	5.4	5.7
NSTAR Electric Company | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.3	0.6	1.1	2.5
Interest Cost	1.9	2.5	5.7	10.3
Expected Return on Pension Plan Assets	(6.6)	(6.4)	(19.9)	(19.2)
Actuarial Loss	0.9	1.2	2.6	1.7
Prior Service Cost/(Credit)	(4.3)	(2.9)	(12.9)	(2.9)
Total Net Periodic Benefit Income	(7.8)	(5.0)	(23.4)	(7.6)
Intercompany Allocations	(0.2)	(0.1)	(0.7)	0.0
Capitalized PBOP Income/ Pension Expense	(4.0)	(2.2)	(11.9)	(3.3)
Public Service Company Of New Hampshire | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	2.4	2.5	7.3	7.5
Interest Cost	5.3	5.1	15.9	15.4
Expected Return on Pension Plan Assets	(10.0)	(9.6)	(29.9)	(28.9)
Actuarial Loss	3.0	2.5	8.7	7.5
Prior Service Cost/(Credit)	0.1	0.1	0.4	0.3
Total Net Periodic Benefit Income	0.8	0.6	2.4	1.8
Intercompany Allocations	0.8	1.0	2.5	3.0
Capitalized PBOP Income/ Pension Expense	0.4	0.4	1.1	1.0
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	0.4	1.0	0.9
Interest Cost	0.8	0.7	2.3	2.2
Expected Return on Pension Plan Assets	(1.4)	(1.4)	(4.1)	(4.2)
Actuarial Loss	0.1	0.2	0.4	0.5
Prior Service Cost/(Credit)	0.2	0.1	0.4	0.1
Total Net Periodic Benefit Income	0.0	0.0	0.0	(0.5)
Intercompany Allocations	(0.1)	0.0	(0.3)	0.0
Capitalized PBOP Income/ Pension Expense	0.0	0.0	0.0	0.0
Western Massachusetts Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.7	0.8	2.3	2.4
Interest Cost	2.1	2.1	6.3	6.3
Expected Return on Pension Plan Assets	(4.4)	(4.4)	(13.3)	(13.1)
Actuarial Loss	1.5	1.3	4.5	4.1
Prior Service Cost/(Credit)	0.1	0.1	0.2	0.2
Total Net Periodic Benefit Income	0.0	(0.1)	0.0	(0.1)
Intercompany Allocations	0.5	0.6	1.4	1.9
Capitalized PBOP Income/ Pension Expense	0.1	0.1	0.3	0.3
Western Massachusetts Electric Company | Other Postretirement Benefits Plan
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	0.8
Expected Return on Pension Plan Assets	(0.6)	(0.6)	(1.7)	(1.7)
Actuarial Loss	0.0	0.0	0.0	0.0
Prior Service Cost/(Credit)	0.0	0.0	0.1	0.0
Total Net Periodic Benefit Income	(0.2)	(0.2)	(0.5)	(0.6)
Intercompany Allocations	0.0	0.0	(0.1)	0.0
Capitalized PBOP Income/ Pension Expense	$ (0.1)	$ (0.1)	$ (0.2)	$ (0.3)