INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Deferred Tax Assets:
Employee Benefits	$ 244.5	$ 228.9
Derivative Liabilities	33.0	53.8
Regulatory Deferrals - Liabilities	452.0	529.5
Allowance for Uncollectible Accounts	143.8	125.5
Tax Effect - Tax Regulatory Liabilities	739.0	762.9
Net Operating Loss Carryforwards	13.8	16.7
Purchase Accounting Adjustment	56.7	62.0
Equity Method Wind Investments	584.9	0.0
Other	200.1	176.6
Total Deferred Tax Assets	2,467.8	1,955.9
Less: Valuation Allowance	328.1	82.8
Net Deferred Tax Assets	2,139.7	1,873.1
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	5,103.3	4,793.7
Property Tax Accruals	95.0	95.3
Regulatory Amounts:
Regulatory Deferrals - Assets	1,512.3	1,251.9
Tax Effect - Tax Regulatory Assets	284.0	271.7
Goodwill-related Regulatory Asset - 1999 Merger	72.2	76.8
Employee Benefits	282.0	305.5
Derivative Assets	6.4	10.5
Other	88.2	135.6
Total Deferred Tax Liabilities	7,443.4	6,941.0
Offshore Wind Business
Regulatory Amounts:
Impairment charge	224.0
CL&P
Deferred Tax Assets:
Employee Benefits	29.9	25.3
Derivative Liabilities	33.0	53.8
Regulatory Deferrals - Liabilities	94.4	146.6
Allowance for Uncollectible Accounts	79.6	60.5
Tax Effect - Tax Regulatory Liabilities	320.7	324.6
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Equity Method Wind Investments	0.0	0.0
Other	125.3	109.5
Total Deferred Tax Assets	682.9	720.3
Less: Valuation Allowance	80.6	63.3
Net Deferred Tax Assets	602.3	657.0
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,703.4	1,602.0
Property Tax Accruals	42.0	40.7
Regulatory Amounts:
Regulatory Deferrals - Assets	470.0	406.4
Tax Effect - Tax Regulatory Assets	191.9	185.6
Goodwill-related Regulatory Asset - 1999 Merger	0.0	0.0
Employee Benefits	38.8	45.0
Derivative Assets	6.4	10.5
Other	9.9	6.8
Total Deferred Tax Liabilities	2,462.4	2,297.0
NSTAR Electric
Deferred Tax Assets:
Employee Benefits	66.8	57.4
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	291.5	285.7
Allowance for Uncollectible Accounts	21.5	20.7
Tax Effect - Tax Regulatory Liabilities	227.1	241.8
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Equity Method Wind Investments	0.0	0.0
Other	25.0	20.5
Total Deferred Tax Assets	631.9	626.1
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	631.9	626.1
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,728.6	1,643.7
Property Tax Accruals	39.8	41.3
Regulatory Amounts:
Regulatory Deferrals - Assets	474.5	407.9
Tax Effect - Tax Regulatory Assets	10.5	10.7
Goodwill-related Regulatory Asset - 1999 Merger	61.9	65.9
Employee Benefits	146.7	140.8
Derivative Assets	0.0	0.0
Other	19.5	16.7
Total Deferred Tax Liabilities	2,481.5	2,327.0
PSNH
Deferred Tax Assets:
Employee Benefits	13.0	11.6
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	23.8	42.1
Allowance for Uncollectible Accounts	3.9	7.9
Tax Effect - Tax Regulatory Liabilities	95.5	97.8
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Equity Method Wind Investments	0.0	0.0
Other	23.4	21.3
Total Deferred Tax Assets	159.6	180.7
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	159.6	180.7
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	566.5	523.8
Property Tax Accruals	6.3	6.7
Regulatory Amounts:
Regulatory Deferrals - Assets	250.3	165.2
Tax Effect - Tax Regulatory Assets	8.3	7.9
Goodwill-related Regulatory Asset - 1999 Merger	0.0	0.0
Employee Benefits	16.8	18.7
Derivative Assets	0.0	0.0
Other	2.9	21.2
Total Deferred Tax Liabilities	$ 851.1	$ 743.5