INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets:
Employee Benefits	$ 602.4	$ 509.4
Derivative Liabilities	92.6	105.0
Regulatory Deferrals - Liabilities	259.8	267.0
Allowance for Uncollectible Accounts	87.5	56.7
Tax Effect - Tax Regulatory Liabilities	810.9	830.4
Net Operating Loss Carryforwards	12.7	9.1
Purchase Accounting Adjustment	54.5	58.7
Other	200.3	190.4
Total Deferred Tax Assets	2,120.7	2,026.7
Less:  Valuation Allowance	48.3	43.0
Net Deferred Tax Assets	2,072.4	1,983.7
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	4,153.6	3,901.0
Property Tax Accruals	88.7	76.8
Regulatory Amounts:
Regulatory Deferrals - Assets	1,376.7	1,155.6
Tax Effect - Tax Regulatory Assets	244.6	238.2
Goodwill Regulatory Asset - 1999 Merger	86.0	90.6
Derivative Assets	17.8	19.7
Other	200.3	257.6
Total Deferred Tax Liabilities	6,167.7	5,739.5
Regulatory liabilities	4,240.2	4,019.2
EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Amounts:
Regulatory liabilities	2,778.6	2,844.6
The Connecticut Light and Power Company
Deferred Tax Assets:
Employee Benefits	144.5	125.4
Derivative Liabilities	91.8	103.6
Regulatory Deferrals - Liabilities	30.2	37.1
Allowance for Uncollectible Accounts	42.3	25.7
Tax Effect - Tax Regulatory Liabilities	331.4	333.5
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	100.9	92.0
Total Deferred Tax Assets	741.1	717.3
Less:  Valuation Allowance	33.7	24.9
Net Deferred Tax Assets	707.4	692.4
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,438.1	1,362.2
Property Tax Accruals	39.0	36.8
Regulatory Amounts:
Regulatory Deferrals - Assets	444.8	340.7
Tax Effect - Tax Regulatory Assets	174.4	171.7
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	17.8	19.7
Other	1.6	5.9
Total Deferred Tax Liabilities	2,115.7	1,937.0
Regulatory liabilities	1,342.1	1,247.8
The Connecticut Light and Power Company | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Amounts:
Regulatory liabilities	1,010.7	1,022.8
NSTAR Electric Company
Deferred Tax Assets:
Employee Benefits	79.8	54.8
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	161.8	165.7
Allowance for Uncollectible Accounts	20.9	17.7
Tax Effect - Tax Regulatory Liabilities	271.8	280.9
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	14.3	35.8
Total Deferred Tax Assets	548.6	554.9
Less:  Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	548.6	554.9
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	1,489.4	1,391.9
Property Tax Accruals	37.0	29.0
Regulatory Amounts:
Regulatory Deferrals - Assets	324.4	276.2
Tax Effect - Tax Regulatory Assets	11.3	11.7
Goodwill Regulatory Asset - 1999 Merger	73.8	77.8
Derivative Assets	0.0	0.0
Other	72.6	125.6
Total Deferred Tax Liabilities	2,008.5	1,912.2
Regulatory liabilities	1,715.2	1,725.8
NSTAR Electric Company | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Amounts:
Regulatory liabilities	1,044.0	1,071.2
Public Service Company of New Hampshire
Deferred Tax Assets:
Employee Benefits	56.6	46.7
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	13.4	19.0
Allowance for Uncollectible Accounts	4.6	2.8
Tax Effect - Tax Regulatory Liabilities	105.2	111.3
Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	19.8	20.0
Total Deferred Tax Assets	199.6	199.8
Less:  Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	199.6	199.8
Deferred Tax Liabilities:
Accelerated Depreciation and Other   Plant-Related Differences	453.8	428.9
Property Tax Accruals	5.8	4.7
Regulatory Amounts:
Regulatory Deferrals - Assets	263.4	260.9
Tax Effect - Tax Regulatory Assets	8.6	8.3
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	5.6	3.2
Total Deferred Tax Liabilities	737.2	706.0
Regulatory liabilities	442.0	479.2
Public Service Company of New Hampshire | EDIT due to Tax Cuts and Jobs Act of 2017
Regulatory Amounts:
Regulatory liabilities	$ 371.5	$ 392.8