INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred Tax Assets:
Employee Benefits	$ 270.8	$ 602.4
Derivative Liabilities	76.8	92.6
Regulatory Deferrals - Liabilities	390.7	259.8
Allowance for Uncollectible Accounts	104.1	87.5
Tax Effect - Tax Regulatory Liabilities	783.4	810.9
Net Operating Loss Carryforwards	7.5	12.7
Purchase Accounting Adjustment	67.2	54.5
Other	196.6	200.3
Total Deferred Tax Assets	1,897.1	2,120.7
Less:  Valuation Allowance	61.5	48.3
Net Deferred Tax Assets	1,835.6	2,072.4
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	4,426.0	4,153.6
Property Tax Accruals	88.1	88.7
Regulatory Amounts:
Regulatory Deferrals - Assets	1,260.3	1,376.7
Tax Effect - Tax Regulatory Assets	257.8	244.6
Goodwill Regulatory Asset - 1999 Merger	81.4	86.0
Derivative Assets	14.9	17.8
Other	304.2	200.3
Total Deferred Tax Liabilities	6,432.7	6,167.7
The Connecticut Light and Power Company
Deferred Tax Assets:
Employee Benefits	23.9	144.5
Derivative Liabilities	76.8	91.8
Regulatory Deferrals - Liabilities	90.9	30.2
Allowance for Uncollectible Accounts	48.8	42.3
Tax Effect - Tax Regulatory Liabilities	328.2	331.4
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	103.9	100.9
Total Deferred Tax Assets	672.5	741.1
Less:  Valuation Allowance	44.5	33.7
Net Deferred Tax Assets	628.0	707.4
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,509.5	1,438.1
Property Tax Accruals	40.5	39.0
Regulatory Amounts:
Regulatory Deferrals - Assets	438.3	444.8
Tax Effect - Tax Regulatory Assets	181.4	174.4
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	14.9	17.8
Other	5.5	1.6
Total Deferred Tax Liabilities	2,190.1	2,115.7
NSTAR Electric Company
Deferred Tax Assets:
Employee Benefits	40.3	79.8
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	215.4	161.8
Allowance for Uncollectible Accounts	21.5	20.9
Tax Effect - Tax Regulatory Liabilities	254.3	271.8
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	21.7	14.3
Total Deferred Tax Assets	553.2	548.6
Less:  Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	553.2	548.6
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,553.7	1,489.4
Property Tax Accruals	33.7	37.0
Regulatory Amounts:
Regulatory Deferrals - Assets	337.6	324.4
Tax Effect - Tax Regulatory Assets	10.9	11.3
Goodwill Regulatory Asset - 1999 Merger	69.9	73.8
Derivative Assets	0.0	0.0
Other	126.9	72.6
Total Deferred Tax Liabilities	2,132.7	2,008.5
Public Service Company of New Hampshire
Deferred Tax Assets:
Employee Benefits	14.1	56.6
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	24.3	13.4
Allowance for Uncollectible Accounts	6.2	4.6
Tax Effect - Tax Regulatory Liabilities	100.9	105.2
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	22.9	19.8
Total Deferred Tax Assets	168.4	199.6
Less:  Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	168.4	199.6
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	482.9	453.8
Property Tax Accruals	6.3	5.8
Regulatory Amounts:
Regulatory Deferrals - Assets	198.4	263.4
Tax Effect - Tax Regulatory Assets	8.3	8.6
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	10.5	5.6
Total Deferred Tax Liabilities	$ 706.4	$ 737.2