INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred Tax Assets:
Employee Benefits	$ 228.9	$ 270.8
Derivative Liabilities	53.8	76.8
Regulatory Deferrals - Liabilities	529.5	390.7
Allowance for Uncollectible Accounts	125.5	104.1
Tax Effect - Tax Regulatory Liabilities	762.9	783.4
Net Operating Loss Carryforwards	16.7	7.5
Purchase Accounting Adjustment	62.0	67.2
Other	176.6	196.6
Total Deferred Tax Assets	1,955.9	1,897.1
Less:  Valuation Allowance	82.8	61.5
Net Deferred Tax Assets	1,873.1	1,835.6
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	4,793.7	4,426.0
Property Tax Accruals	95.3	88.1
Regulatory Amounts:
Regulatory Deferrals - Assets	1,251.9	1,260.3
Tax Effect - Tax Regulatory Assets	271.7	257.8
Goodwill-related Regulatory Asset - 1999 Merger	76.8	81.4
Employee Benefits	305.5	174.7
Derivative Assets	10.5	14.9
Other	135.6	129.5
Total Deferred Tax Liabilities	6,941.0	6,432.7
CL&P
Deferred Tax Assets:
Employee Benefits	25.3	23.9
Derivative Liabilities	53.8	76.8
Regulatory Deferrals - Liabilities	146.6	90.9
Allowance for Uncollectible Accounts	60.5	48.8
Tax Effect - Tax Regulatory Liabilities	324.6	328.2
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	109.5	103.9
Total Deferred Tax Assets	720.3	672.5
Less:  Valuation Allowance	63.3	44.5
Net Deferred Tax Assets	657.0	628.0
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,602.0	1,509.5
Property Tax Accruals	40.7	40.5
Regulatory Amounts:
Regulatory Deferrals - Assets	406.4	438.3
Tax Effect - Tax Regulatory Assets	185.6	181.4
Goodwill-related Regulatory Asset - 1999 Merger	0.0	0.0
Employee Benefits	45.0	3.5
Derivative Assets	10.5	14.9
Other	6.8	2.0
Total Deferred Tax Liabilities	2,297.0	2,190.1
NSTAR Electric
Deferred Tax Assets:
Employee Benefits	57.4	40.3
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	285.7	215.4
Allowance for Uncollectible Accounts	20.7	21.5
Tax Effect - Tax Regulatory Liabilities	241.8	254.3
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	20.5	21.7
Total Deferred Tax Assets	626.1	553.2
Less:  Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	626.1	553.2
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,643.7	1,553.7
Property Tax Accruals	41.3	33.7
Regulatory Amounts:
Regulatory Deferrals - Assets	407.9	337.6
Tax Effect - Tax Regulatory Assets	10.7	10.9
Goodwill-related Regulatory Asset - 1999 Merger	65.9	69.9
Employee Benefits	140.8	107.1
Derivative Assets	0.0	0.0
Other	16.7	19.8
Total Deferred Tax Liabilities	2,327.0	2,132.7
PSNH
Deferred Tax Assets:
Employee Benefits	11.6	14.1
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	42.1	24.3
Allowance for Uncollectible Accounts	7.9	6.2
Tax Effect - Tax Regulatory Liabilities	97.8	100.9
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	21.3	22.9
Total Deferred Tax Assets	180.7	168.4
Less:  Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	180.7	168.4
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	523.8	482.9
Property Tax Accruals	6.7	6.3
Regulatory Amounts:
Regulatory Deferrals - Assets	165.2	198.4
Tax Effect - Tax Regulatory Assets	7.9	8.3
Goodwill-related Regulatory Asset - 1999 Merger	0.0	0.0
Employee Benefits	18.7	0.7
Derivative Assets	0.0	0.0
Other	21.2	9.8
Total Deferred Tax Liabilities	$ 743.5	$ 706.4