INCOME TAXES - Temporary Differences That Give Rise To Accumulated Deferred Income Tax Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Employee Benefits	$ 442.1	$ 640.6
Derivative Liabilities	111.8	192.6
Regulatory Deferrals - Liabilities	205.6	290.9
Allowance for Uncollectible Accounts	50.1	76.6
Tax Effect - Tax Regulatory Liabilities	832.6	11.8
Federal Net Operating Loss Carryforwards	47.8	0.0
Purchase Accounting Adjustment	69.9	112.2
Other	149.5	170.5
Total Deferred Tax Assets	1,909.4	1,495.2
Less: Valuation Allowance	14.6	5.1
Net Deferred Tax Assets	1,894.8	1,490.1
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	3,562.0	5,001.2
Property Tax Accruals	56.7	81.9
Regulatory Amounts:
Regulatory Deferrals - Assets	924.9	1,321.8
Tax Effect - Tax Regulatory Assets	243.1	252.6
Goodwill Regulatory Asset - 1999 Merger	99.8	186.7
Derivative Assets	17.4	29.5
Other	288.4	223.6
Total Deferred Tax Liabilities	5,192.3	7,097.3
The Connecticut Light And Power Company
Deferred Tax Assets:
Employee Benefits	112.3	138.8
Derivative Liabilities	110.5	191.5
Regulatory Deferrals - Liabilities	12.0	6.3
Allowance for Uncollectible Accounts	20.6	33.0
Tax Effect - Tax Regulatory Liabilities	337.2	4.9
Other	70.7	59.4
Total Deferred Tax Assets	663.3	433.9
Less: Valuation Allowance	6.3	4.5
Net Deferred Tax Assets	657.0	429.4
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,224.9	1,700.3
Property Tax Accruals	20.7	29.7
Regulatory Amounts:
Regulatory Deferrals - Assets	310.6	473.4
Tax Effect - Tax Regulatory Assets	173.1	170.4
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	17.4	27.0
Other	13.7	16.3
Total Deferred Tax Liabilities	1,760.4	2,417.1
NSTAR Electric Company
Deferred Tax Assets:
Employee Benefits	34.0	69.5
Derivative Liabilities	0.3	1.1
Regulatory Deferrals - Liabilities	139.8	194.9
Allowance for Uncollectible Accounts	17.3	25.7
Tax Effect - Tax Regulatory Liabilities	281.2	3.3
Other	4.9	6.6
Total Deferred Tax Assets	477.5	301.1
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	477.5	301.1
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	1,229.2	1,901.9
Property Tax Accruals	24.2	36.8
Regulatory Amounts:
Regulatory Deferrals - Assets	267.1	381.7
Tax Effect - Tax Regulatory Assets	9.8	44.8
Goodwill Regulatory Asset - 1999 Merger	85.7	160.3
Derivative Assets	0.0	0.0
Other	137.3	102.7
Total Deferred Tax Liabilities	1,753.3	2,628.2
Public Service Company Of New Hampshire
Deferred Tax Assets:
Employee Benefits	38.0	46.5
Derivative Liabilities	0.0	0.0
Regulatory Deferrals - Liabilities	17.9	36.7
Allowance for Uncollectible Accounts	2.9	4.1
Tax Effect - Tax Regulatory Liabilities	116.8	2.6
Other	49.6	56.4
Total Deferred Tax Assets	225.2	146.3
Less: Valuation Allowance	0.0	0.0
Net Deferred Tax Assets	225.2	146.3
Deferred Tax Liabilities:
Accelerated Depreciation and Other Plant-Related Differences	502.5	726.3
Property Tax Accruals	5.5	8.0
Regulatory Amounts:
Regulatory Deferrals - Assets	103.6	142.1
Tax Effect - Tax Regulatory Assets	11.4	12.2
Goodwill Regulatory Asset - 1999 Merger	0.0	0.0
Derivative Assets	0.0	0.0
Other	45.7	43.1
Total Deferred Tax Liabilities	$ 668.7	$ 931.7