INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets:
Employee Benefits	$ 388.2	$ 442.1
Derivative Liabilities	111.4	111.8
Regulatory Deferrals - Liabilities	299.3	205.6
Allowance for Uncollectible Accounts	54.0	50.1
Tax Effect - Tax Regulatory Liabilities	830.3	832.6
Federal Net Operating Loss Carryforwards	28.5	47.8
Purchase Accounting Adjustment	64.2	69.9
Other	166.2	149.5
Total Deferred Tax Assets	1,942.1	1,909.4
Less: Valuation Allowance	19.5	14.6
Net Deferred Tax Assets	1,922.6	1,894.8
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	3,724.2	3,562.0
Property Tax Accruals	73.2	56.7
Regulatory Amounts:
Regulatory Deferrals - Assets	1,025.9	924.9
Tax Effect - Tax Regulatory Assets	238.9	243.1
Goodwill Regulatory Asset - 1999 Merger	95.2	99.8
Derivative Assets	20.1	17.4
Other	251.1	288.4
Total Deferred Tax Liabilities	5,428.6	5,192.3
Regulatory Liabilities	3,979.7	3,765.4
Refunds to customers	5.0
The Connecticut Light And Power Company
Deferred Tax Assets:
Employee Benefits	94.5	112.3
Derivative Liabilities	111.4	110.5
Regulatory Deferrals - Liabilities	38.6	12.0
Allowance for Uncollectible Accounts	23.1	20.6
Tax Effect - Tax Regulatory Liabilities	336.8	337.2
Federal Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	81.1	70.7
Total Deferred Tax Assets	685.5	663.3
Less: Valuation Allowance	10.7	6.3
Net Deferred Tax Assets	674.8	657.0
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,293.3	1,224.9
Property Tax Accruals	35.4	20.7
Regulatory Amounts:
Regulatory Deferrals - Assets	320.1	310.6
Tax Effect - Tax Regulatory Assets	167.0	173.1
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	19.9	17.4
Other	5.9	13.7
Total Deferred Tax Liabilities	1,841.6	1,760.4
Regulatory Liabilities	1,231.8	1,151.1
NSTAR Electric Company
Deferred Tax Assets:
Employee Benefits	35.0	34.0
Derivative Liabilities	0.0	0.3
Regulatory Deferrals - Liabilities	195.5	139.8
Allowance for Uncollectible Accounts	17.8	17.3
Tax Effect - Tax Regulatory Liabilities	288.9	281.2
Federal Net Operating Loss Carryforwards	0.0	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	15.6	4.9
Total Deferred Tax Assets	552.8	477.5
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	552.8	477.5
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,342.4	1,229.2
Property Tax Accruals	26.3	24.2
Regulatory Amounts:
Regulatory Deferrals - Assets	277.4	267.1
Tax Effect - Tax Regulatory Assets	9.7	9.8
Goodwill Regulatory Asset - 1999 Merger	81.7	85.7
Derivative Assets	0.0	0.0
Other	109.8	137.3
Total Deferred Tax Liabilities	1,847.3	1,753.3
Regulatory Liabilities	1,703.9	1,594.1
Public Service Company Of New Hampshire
Deferred Tax Assets:
Employee Benefits	31.1	38.0
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	16.1	17.9
Allowance for Uncollectible Accounts	3.0	2.9
Tax Effect - Tax Regulatory Liabilities	111.7	116.8
Federal Net Operating Loss Carryforwards	0.6	0.0
Purchase Accounting Adjustment	0.0	0.0
Other	33.4	49.6
Total Deferred Tax Assets	195.9	225.2
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	195.9	225.2
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	410.6	502.5
Property Tax Accruals	5.2	5.5
Regulatory Amounts:
Regulatory Deferrals - Assets	213.8	103.6
Tax Effect - Tax Regulatory Assets	8.1	11.4
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	39.4	45.7
Total Deferred Tax Liabilities	677.1	668.7
Regulatory Liabilities	483.6	$ 450.7
Refunds to customers	4.4
Yankee Gas Services Company
Regulatory Amounts:
Refunds to customers	$ 0.6