EMPLOYEE BENEFITS - Schedule of Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 75.0	$ 91.4	$ 79.9
Interest Cost	185.5	227.0	225.7
Expected Return on Pension Plan Assets	(317.9)	(335.9)	(310.8)
Actuarial Loss/(Gain)	125.7	148.5	128.4
Prior Service Cost	3.6	3.7	4.4
Total Net Periodic Benefit Expense/(Income)	71.9	134.7	127.6
Capitalized PBOP Expense/(Income)	(22.1)	(41.0)	(35.2)
Amount included in other deferred debits		3.2
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	12.2	16.3	12.5
Interest Cost	32.9	47.2	49.5
Expected Return on Pension Plan Assets	(62.9)	(67.4)	(63.3)
Actuarial Loss/(Gain)	9.0	6.8	12.2
Prior Service Cost	(9.1)	(0.5)	(2.8)
Total Net Periodic Benefit Expense/(Income)	(17.9)	2.4	8.1
Capitalized PBOP Expense/(Income)	(8.0)	0.1	(1.4)
The Connecticut Light And Power Company [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	18.8	24.7	20.2
Interest Cost	41.6	51.1	50.5
Expected Return on Pension Plan Assets	(72.1)	(78.9)	(75.4)
Actuarial Loss/(Gain)	25.4	32.2	33.7
Prior Service Cost	1.5	1.5	1.8
Total Net Periodic Benefit Expense/(Income)	15.2	30.6	30.8
Intercompany Allocations	13.8	22.5	26.7
Capitalized PBOP Expense/(Income)	(9.3)	(18.8)	(17.6)
The Connecticut Light And Power Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2.0	2.1	2.2
Interest Cost	5.3	7.2	8.1
Expected Return on Pension Plan Assets	(10.1)	(11.1)	(10.5)
Actuarial Loss/(Gain)	1.5	0.7	4.2
Prior Service Cost	0.5	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(0.8)	(1.1)	4.0
Intercompany Allocations	0.3	1.9	3.8
Capitalized PBOP Expense/(Income)	(0.5)	(0.2)	(1.8)
NSTAR Electric Company [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	13.2	14.9	13.6
Interest Cost	33.8	40.2	41.3
Expected Return on Pension Plan Assets	(67.6)	(70.0)	(63.0)
Actuarial Loss/(Gain)	34.4	35.8	23.5
Prior Service Cost	0.0	(0.1)	0.0
Total Net Periodic Benefit Expense/(Income)	13.8	20.8	15.4
Intercompany Allocations	8.9	13.6	10.4
Capitalized PBOP Expense/(Income)	(7.6)	(11.4)	(7.9)
NSTAR Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	3.0	5.4	3.1
Interest Cost	12.2	19.0	19.4
Expected Return on Pension Plan Assets	(25.7)	(27.3)	(25.9)
Actuarial Loss/(Gain)	3.2	2.3	(0.5)
Prior Service Cost	(7.2)	(0.2)	(1.9)
Total Net Periodic Benefit Expense/(Income)	(14.5)	(0.8)	(5.8)
Intercompany Allocations	(0.2)	0.8	0.8
Capitalized PBOP Expense/(Income)	(6.4)	(0.2)	2.3
Public Service Company Of New Hampshire [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	9.9	12.1	9.7
Interest Cost	20.7	24.3	23.8
Expected Return on Pension Plan Assets	(38.6)	(40.4)	(38.1)
Actuarial Loss/(Gain)	9.9	11.6	11.6
Prior Service Cost	0.5	0.5	0.7
Total Net Periodic Benefit Expense/(Income)	2.4	8.1	7.7
Intercompany Allocations	4.0	6.7	7.6
Capitalized PBOP Expense/(Income)	(1.4)	(3.5)	(3.0)
Public Service Company Of New Hampshire [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	1.3	1.4	1.3
Interest Cost	2.9	3.9	4.3
Expected Return on Pension Plan Assets	(5.5)	(6.0)	(5.4)
Actuarial Loss/(Gain)	0.7	0.5	2.2
Prior Service Cost	0.2	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(0.4)	(0.2)	2.4
Intercompany Allocations	(0.1)	0.4	1.0
Capitalized PBOP Expense/(Income)	0.1	0.2	(0.8)
Western Massachusetts Electric Company [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	3.1	4.3	3.5
Interest Cost	8.4	10.4	10.3
Expected Return on Pension Plan Assets	(17.5)	(18.9)	(17.9)
Actuarial Loss/(Gain)	5.5	6.4	6.9
Prior Service Cost	0.3	0.3	0.4
Total Net Periodic Benefit Expense/(Income)	(0.2)	2.5	3.2
Intercompany Allocations	2.5	4.4	5.1
Capitalized PBOP Expense/(Income)	(0.4)	(1.9)	(2.4)
Western Massachusetts Electric Company [Member] | Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost	0.4	0.4	0.4
Interest Cost	1.1	1.5	1.7
Expected Return on Pension Plan Assets	(2.4)	(2.5)	(2.3)
Actuarial Loss/(Gain)	0.1	0.0	0.5
Prior Service Cost	0.1	0.0	0.0
Total Net Periodic Benefit Expense/(Income)	(0.7)	(0.6)	0.3
Intercompany Allocations	0.1	0.3	0.7
Capitalized PBOP Expense/(Income)	$ (0.3)	$ (0.2)	$ (0.2)