INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Deferred Tax Assets:
Employee Benefits	$ 273.0	$ 266.9
Derivative Liabilities	205.8	19.6
Regulatory Deferrals - Liabilities	716.7	508.7
Allowance for Uncollectible Accounts	151.9	145.3
Tax Effect - Tax Regulatory Liabilities	696.0	720.8
Net Operating Loss Carryforwards	314.0	198.2
Purchase Accounting Adjustment	48.2	52.1
Equity Method Wind Investments	760.3	1,098.6
Other	229.6	267.1
Total Deferred Tax Assets	3,395.5	3,277.3
Less: Valuation Allowance	293.4	558.2
Net Deferred Tax Assets	3,102.1	2,719.1
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	5,880.4	5,493.3
Property Tax Accruals	118.3	107.4
Regulatory Amounts:
Regulatory Deferrals - Assets	1,837.0	1,709.1
Tax Effect - Tax Regulatory Assets	308.5	294.5
Goodwill-related Regulatory Asset - 1999 Merger	62.9	67.5
Employee Benefits	404.1	353.0
Derivative Assets	24.9	3.8
Other	113.2	101.7
Total Deferred Tax Liabilities	8,749.3	8,130.3
Tax Equity Investment in South Fork Wind
Regulatory Amounts:
Impairment charge	162.4	427.0
CL&P
Deferred Tax Assets:
Employee Benefits	37.3	33.7
Derivative Liabilities	0.0	19.6
Regulatory Deferrals - Liabilities	212.7	128.0
Allowance for Uncollectible Accounts	70.2	75.4
Tax Effect - Tax Regulatory Liabilities	310.7	317.6
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Equity Method Wind Investments	0.0	0.0
Other	133.3	134.7
Total Deferred Tax Assets	764.2	709.0
Less: Valuation Allowance	104.2	104.1
Net Deferred Tax Assets	660.0	604.9
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,925.0	1,820.3
Property Tax Accruals	56.8	49.8
Regulatory Amounts:
Regulatory Deferrals - Assets	405.1	522.9
Tax Effect - Tax Regulatory Assets	199.6	194.7
Goodwill-related Regulatory Asset - 1999 Merger	0.0	0.0
Employee Benefits	60.5	51.0
Derivative Assets	0.0	3.8
Other	18.9	15.2
Total Deferred Tax Liabilities	2,665.9	2,657.7
NSTAR Electric
Deferred Tax Assets:
Employee Benefits	92.5	83.5
Derivative Liabilities	205.8	0.0
Regulatory Deferrals - Liabilities	402.5	316.7
Allowance for Uncollectible Accounts	30.7	26.5
Tax Effect - Tax Regulatory Liabilities	201.1	214.3
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Equity Method Wind Investments	0.0	0.0
Other	30.9	31.5
Total Deferred Tax Assets	963.5	672.5
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	963.5	672.5
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,963.2	1,845.2
Property Tax Accruals	47.5	44.5
Regulatory Amounts:
Regulatory Deferrals - Assets	725.3	519.9
Tax Effect - Tax Regulatory Assets	9.6	10.0
Goodwill-related Regulatory Asset - 1999 Merger	54.0	58.0
Employee Benefits	194.7	175.4
Derivative Assets	24.9	0.0
Other	31.6	24.9
Total Deferred Tax Liabilities	3,050.8	2,677.9
PSNH
Deferred Tax Assets:
Employee Benefits	16.1	14.5
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	48.3	19.6
Allowance for Uncollectible Accounts	6.2	3.8
Tax Effect - Tax Regulatory Liabilities	90.5	93.2
Net Operating Loss Carryforwards	0.0	1.5
Purchase Accounting Adjustment	0.0	0.0
Equity Method Wind Investments	0.0	0.0
Other	19.7	24.1
Total Deferred Tax Assets	180.8	156.7
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	180.8	156.7
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	673.3	618.4
Property Tax Accruals	7.0	6.5
Regulatory Amounts:
Regulatory Deferrals - Assets	263.4	277.8
Tax Effect - Tax Regulatory Assets	10.3	9.4
Goodwill-related Regulatory Asset - 1999 Merger	0.0	0.0
Employee Benefits	31.0	23.5
Derivative Assets	0.0	0.0
Other	2.1	2.7
Total Deferred Tax Liabilities	$ 987.1	$ 938.3