INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets:
Employee Benefits	$ 509.4	$ 388.2
Derivative Liabilities	105.0	111.4
Regulatory Deferrals - Liabilities	267.0	299.3
Allowance for Uncollectible Accounts	56.7	54.0
Tax Effect - Tax Regulatory Liabilities	830.4	830.3
Federal Net Operating Loss Carryforwards	9.1	28.5
Purchase Accounting Adjustment	58.7	64.2
Other	190.4	166.2
Total Deferred Tax Assets	2,026.7	1,942.1
Less: Valuation Allowance	43.0	19.5
Net Deferred Tax Assets	1,983.7	1,922.6
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	3,901.0	3,724.2
Property Tax Accruals	76.8	73.2
Regulatory Amounts:
Regulatory Deferrals - Assets	1,155.6	1,025.9
Tax Effect - Tax Regulatory Assets	238.2	238.9
Goodwill Regulatory Asset - 1999 Merger	90.6	95.2
Derivative Assets	19.7	20.1
Other	257.6	251.1
Total Deferred Tax Liabilities	5,739.5	5,428.6
Regulatory liabilities	4,019.2	3,979.7
Refunds to customers	51.5
EDIT due to Tax Cuts and Jobs Act
Regulatory Amounts:
Regulatory liabilities	2,844.6	2,883.0
CL&P
Deferred Tax Assets:
Employee Benefits	125.4	94.5
Derivative Liabilities	103.6	111.4
Regulatory Deferrals - Liabilities	37.1	38.6
Allowance for Uncollectible Accounts	25.7	23.1
Tax Effect - Tax Regulatory Liabilities	333.5	336.8
Federal Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	92.0	81.1
Total Deferred Tax Assets	717.3	685.5
Less: Valuation Allowance	24.9	10.7
Net Deferred Tax Assets	692.4	674.8
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,362.2	1,293.3
Property Tax Accruals	36.8	35.4
Regulatory Amounts:
Regulatory Deferrals - Assets	340.7	320.1
Tax Effect - Tax Regulatory Assets	171.7	167.0
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	19.7	19.9
Other	5.9	5.9
Total Deferred Tax Liabilities	1,937.0	1,841.6
Regulatory liabilities	1,247.8	1,231.8
Refunds to customers	7.9
CL&P | EDIT due to Tax Cuts and Jobs Act
Regulatory Amounts:
Regulatory liabilities	1,022.8	1,031.0
NSTAR Electric
Deferred Tax Assets:
Employee Benefits	54.8	35.0
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	165.7	195.5
Allowance for Uncollectible Accounts	17.7	17.8
Tax Effect - Tax Regulatory Liabilities	280.9	288.9
Federal Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	35.8	15.6
Total Deferred Tax Assets	554.9	552.8
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	554.9	552.8
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,391.9	1,342.4
Property Tax Accruals	29.0	26.3
Regulatory Amounts:
Regulatory Deferrals - Assets	276.2	277.4
Tax Effect - Tax Regulatory Assets	11.7	9.7
Goodwill Regulatory Asset - 1999 Merger	77.8	81.7
Derivative Assets	0.0	0.0
Other	125.6	109.8
Total Deferred Tax Liabilities	1,912.2	1,847.3
Regulatory liabilities	1,725.8	1,703.9
Refunds to customers	31.5
NSTAR Electric | EDIT due to Tax Cuts and Jobs Act
Regulatory Amounts:
Regulatory liabilities	1,071.2	1,103.7
PSNH
Deferred Tax Assets:
Employee Benefits	46.7	31.1
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	19.0	16.1
Allowance for Uncollectible Accounts	2.8	3.0
Tax Effect - Tax Regulatory Liabilities	111.3	111.7
Federal Net Operating Loss Carryforwards	0.0	0.6
Purchase Accounting Adjustment	0.0	0.0
Other	20.0	33.4
Total Deferred Tax Assets	199.8	195.9
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	199.8	195.9
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	428.9	410.6
Property Tax Accruals	4.7	5.2
Regulatory Amounts:
Regulatory Deferrals - Assets	260.9	213.8
Tax Effect - Tax Regulatory Assets	8.3	8.1
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	3.2	39.4
Total Deferred Tax Liabilities	706.0	677.1
Regulatory liabilities	479.2	483.6
Refunds to customers	5.6
PSNH | EDIT due to Tax Cuts and Jobs Act
Regulatory Amounts:
Regulatory liabilities	392.8	$ 396.4
Yankee Gas Services Company
Regulatory Amounts:
Refunds to customers	1.4
NSTAR Gas
Regulatory Amounts:
Refunds to customers	$ 5.1